|"ATI HD 2400pro"|
That's a good chipset for a DirectX 10 compatible cheaper card that works well with most recent games, according to user reviews - I have the Powercolor PCI version myself.
If you have installed Windows from a regular CD, you must load the main chipset drivers if you haven't already done so, otherwise the AGP support of the mboard chipset may not be detected properly by XP. If you're not sure whether they have been loaded, it usually does no harm at all to load them again if they are already present, if the main chipset is not NVidia (NVidia often uses a huge all in one drivers download that may require you un-install some stuff first).
Whenever you load Windows from a regular Windows CD (or DVD) from scratch, after Setup is finished you must load the drivers for the mboard, particularly the main chipset drivers, in order for Windows to have the proper drivers for and information about your mboard hardware, including it's AGP or PCI-E, ACPI, USB 2.0 if it has it, and hard drive controller support. If you have a generic system and have the CD that came with the mboard, all the necessary drivers are on it. If you load drivers from the web, brand name system builders and mboard makers often DO NOT have the main chipset drivers listed in the downloads for your model - in that case you must go to the maker of the main chipset's web site, get the drivers, and load them.
You could try unplugging or switching off the AC power to the computer, removing the video card, wiping it's contacts with at least a tissue or paper towel or soft cloth, or better, wipe them off with methyl or isopropyl (rubbing) alcohol or a drop of dish washing detergent in water, wipe them dry, don't touch the contacts with your fingers after having cleaned them,........
How long as this computer been without a video card in the AGP slot? If it's been a while, examine the AGP slot to see if it has crap in it - you will need good lighting, maybe a magnifying device - if it does have crap in it, blow it out - don't use a vaccum cleaner - they produce a tremendous amount of static electricity when running - wrap a tissue or paper towel around a piece of thin cardboard, wipe the contacts in the slot - a drop or two of methyl or isopropyl alcohol on it is better.
......install the card, make sure it's all the way down, fasten it down, restore AC power.
Go into the bios Setup and set Primary Video or Initialize Video First or similar to AGP, save settings.
The video will still work in XP if that is wrong, but the enhanced AGP capabilities will not work properly in XP once the specific drivers for the chipset have been loaded if that's wrong.
DO NOT install video drivers (or most drivers for that matter) while booting Windows BEFORE the proper software has been installed by it's proper installation program on the CD, or by the proper download you got from the web.
Go to Control Panel - Add/Remove Programs and un-install any listings for the card's software, or any ATI software, especially if ATI graphics related software is listed. Sometimes there is a "master" ATI entry that installs all or most of that. If there is a listing for Catalyst, un-install that too.
After that has been removed, if applicable....
It's a good idea to disable any resident module (a part that runs all the time and scans for suspicious activity) of any anti-malware software you have running at this point.
E.g. free or paid AVG, Norton Products, Avast! free or paid, Spybot free or paid, AdAware paid, Malwarebytes paid have a resident module.
AdAware free, Malwarebytes free do not have a resident module.
If you don't know whether that applies or how to do that, tell us which anti-malware program(s) you're using, or look in it's/their Help.
Go to Device Manager (e.g. RIGHT click on My Computer - Properties - Hardware - Device Manager) - Display Adapters.
In most cases the onboard video, if your mboard has that, is automatically disabled when you install an AGP card in a slot - it should NOT be listed in most cases. If there is a listing for a default VGA adapter of some sort, or a pair of them, that's fine. If anything other than that is listed, RIGHT click on it and Un-install it - DO NOT reboot at that point if you did that.
Install the drivers for the card using the proper CD installation program or download . THEN reboot the computer.
If you disabled any resident module(s) of (an) anti- malware program(s), enable it (them) again.
NOTE that if you download the ATI drivers from the web, if you are installing Catayst, it requires a Microsoft .Net Framework version to be installed before Catalyst is installed. The specific .Net Framework version required (usually 2.0) is specified in the release notes or ReadMe file for the Catalyst version on the ATI web site. If that version has already been installed, it's listed in Control Panel - Add/Remove Programs, or it's update is listed (SP2?) . If it wasn't already installed, you may get a situation where after you have installed it, a Logon screen appears while booting when one didn't appear previously. If you go to the Microsoft web site to Microsoft Update or Windows Update, and do an Express search, it will find updates for the .Net Framework version that will cure the logon screen when there wasn't one before problem, or you can just let Automatic Update install that eventually.
When you install the software from the CD that came with the card, it installs the needed .Net Framework version if it isn't already present in Windows, if you choose to install Catalyst.
If you have any problems with the software installed from the CD, which is quite rare, ATI is pretty good about removing buggy software versions from their web site - uninstall the ATI software the CD installed, install the downloads you get from the web site (AMD.com - ATI merged with AMD several years ago).
Note that if you install display drivers and Catalyst their versions are matched to each other - the display drivers version changes more often than the Catayst version. If you install newer display drivers without installing the Catalyst version matched with it, if the display drivers version is not listed along with the Catalyst version you already have on the computer on the ATI web site, you may have problems with Catalyst, and will need to un-install it and install the newer Catalyst version listed on the ATI web site (AMD.com) .(That's probably the most frequent reason people have problems with Catalyst.)