|"I have since installed service pack 2, and so the system says I cannot install an older version...."|
You only get that message when you try to run Setup in Windows.
You can't install Windows from scratch, or run a Repair installation of XP, on the same partition you are running Windows from in any case.
".... but that I can boot from the CD. But when I set the PC to boot from the CD, it keeps going to the Hard Disk instead!"
"The problem was that I was using a usb wireless mouse and keyboard, so although it let me select the CD to boot from, it then didn't recognise 'any key'!! Foolish me. I have now connected an old keyboard - and all going well."
You didn't mention you were getting the "Press any key to boot from CD" or similar line while booting but there was no response when you pressed the key and it then loaded the hard drive's Windows,.or whatever operating system, installation.
(You don't get that "Press any key to boot from CD" or similar line while booting if the bios detects no bootable hard drive partition on the first hard drive the bios detects - a bootable CD or DVD will boot automatically if the bios Boot Order or similar settings are set correctly to boot from a bootable disk in an optical drive if it's detected it's bootable.
Also, some newer mboard CDs or DVDs for retail mboard models, e.g. Asus models, will automatically load (a program from the CD or DVD that allows you to possibly recover from a failed bios flash) even when the bios DOES detect a bootable hard drive partition on the first hard drive the bios detects, if the CD or DVD is already in an optical drive while booting the computer. )
At least some bioses do not recognize wireless keyboards when you press the key to do that - maybe all of them ? - I don't have a wireless keyboard to try that with.
Also, in order to be able to get into the bios with a USB corded keyboard, the setting Legacy USB or USB keyboard or similar must be enabled in the bios Setup.
In older bioses that may not be enabled by default. In newer bioses it usually is enabled by default. I don't know if that applies to wireless keyboards too (they use a USB wireless transmitter / receiver device) .
A PS/2 keyboard always works for getting into the bios - it must be plugged into the proper PS/2 port of course, in most cases - it's usually colored purple.
If you use a simple gender adapter with a corded USB keyboard to convert it to PS/2 port use, the keyboard must be a "combo" keyboard meant to be used with both types of ports, and the gender adapter often must been one meant for use with a "combo" keyboard - usually colored purple - one meant for a "combo" mouse - usually colored green - will probably NOT work with a "combo" keyboard.
"....the PC is useless with whatever virus is infecting it."
You usually DO NOT need to install Windows from scratch in order to remove the malware. Most malware these days is NOT a virus.
If you had described what your symptoms were we could have probably told you what to try using to get rid if it. In most cases that's 100% successful , if not right away, after you use more than one anti-malware program.
Running Setup from scratch is a last resort, and it's only the first step of MANY you'll need to do.- it can take DAYS to get Windows fully updated and everything else required and that you want on the computer installed. .
"From the sound of your post you are trying to do an upgrade or repair using an pre-SP1 CD? This won't work for the reasons you mention in your post. Even if you created a slipstreamed SP2 disk (which would get around the problem) it doesn't necessarily remove all the malware on your system.'
"Incorrect" He's right.
In addition to what he said...
If the partition the operating system was installed on is deleted and a new one is made at the beginning of Setup to install Windows on, if it was the only hard drive partition that malware could have been on, the malware will NOT be present anymore, unless the malware has infected the master boot record of the hard drive or the bios of the mboard, both of which are extremely unlikely these days.
In most cases, ALL the malware is / was ONLY on the hard drive partition the operating system loaded from.
If you boot the computer FROM an XP CD that is not an Upgrade version (which is usually if not always not bootable), it doesn't matter whether the XP CD has no SP updates, SP1 updates, SP2 updates, or SP3 updates on it in order for you to be able to install XP from scratch. You can install SP3 updates later after Setup has finished after you or Automatic Update have / has loaded Windows updates that are required to be loaded before you can install SP3 updates.
(If the XP CD has no SP Windows updates at all, you have to install SP1 or SP2 updates BEFORE you can install SP3 updates [ That's NOT required when you make a "slipstreamed" CD.] Automatic Update does NOT install SP1, SP2, or SP3 Windows updates automatically, but it will download them automatically if it's set to default or other suitable settings, eventually.)
However, the CD must have SP1 or later Windows updates integrated into it in order for Setup to be able to recognize the full size of hard drives larger than 137 gb hard drive manufacturer's size (= larger than 128 gb binary in the bios and in Windows ). (The mboard's bios must also recognize the full size of hard drives larger than 137 gb hard drive manufacturer's size - all mboards made in about 2001 or later do.)
If you DO have hard drives larger than 137 gb hard drive manufacturer's size, and your XP CD has no SP Windows updates integrated into it at all......
- the drive will be detected as ~ 128 gb (binary size) by Setup, and by Windows.
You can't do anything about that until at least SP1 Windows updates have been installed in Windows, and even then you must use a third party program such as the freeware Easeus Partition Master Home Edition if you want to add the space that is available to another partition, if you don't want to lose the existing data on that partition.
- you can make yourself a "slipstreamed CD, a CD-R disk will work with virtually all computers [that have a built in conventional optical drive - the XP CDs can only recognize a small number of USB optical drive models that were avalable when XP was first released], that has had the SP3 updates integrated into the contents of your original XP CD by you following a procedure described in many places on the web, then use that "slipstreamed" CD instead of your original XP CD to install Windows, along with your original Product Key. If your mboard has SATA drive controllers, you can also integrate the drivers for those into the CD while you're at it, e.g. by using the freeware nLite program to do both things.
Also - the support for USB 2.0 controllers and USB 2.0 devices is not present in XP unless or until the XP CD or Windows has least SP1 Windows updates included or installed.When SP1 or later SP Windows updates have been installed, the built in support for USB 2.0 is not installed in Windows until AFTER you have installed the main chipset drivers for your mboard in Windows after XP's Setup has finiished,.
"You need to boot from the CD & do the install. It doesn't matter which version of Windows, Linux, etc is on the HDD at the time because it will be completely removed & replaced with a new OS"
If you have more than one partition on the hard drive(s), you have the option of only replacing the data on the one partition the operating system was running from previously.
"Hopefully you realize that by doing this, you will lose EVERYTHING. All files, pictures, videos, etc will be deleted. And you will end up spending several hours installing SP3, IE8, all the related Windows updates, system drivers, anti-virus & any other programs you need/use. It is by no means a small undertaking."
If you have personal data you DO NOT want to lose, if Windows or whatever operating system is NOT working properly, you can boot the computer from somethinmg such as a Linux CD that has an operating system that can read all the files on your hard drive and be able to copy the data you don't want to lose to elsewhere BEFORE you boot the Windows CD and install Windows from scratch.
In XP, ALL of your personal data is saved at (drive letter of the Windows partition that was loaded):\Documents and Settings\(your user name)\(your user name's files, sub-folders, the files in those sub-folders) , unless you chose a non-default location to save it to, elsewhere.
- or - you can remove your hard drive and connect to another computer that has a working operating system that can read all the files on your hard drive and do the same thing on the other computer, then remove it and install it in your computer again, then boot the from the Windows CD