|"how do i know which drivers to download?"|
They're in the downloads for the specific compaq model on the HP web site.
Or, if you have a disk with drivers on it for the model that came with it when it was new, they would be on that.
Something with a description that says SATA drivers for a floppy disk, or for when you press F6.
For XP, the floppy drive usually must be built into the computer - the files initially loaded by the XP CD cannot recognize most USB floppy drive models, unless the model is an old one that was available when XP first came out circa 2001. The XP CD cannot recognize the SATA drivers source being on anything other than a floppy disk, unless they've been integrated into Windows on a "slipstreamed" CD you have made.
(The Vista or Windows 7 DVD is able to recognize the SATA drivers source being on other things such as a flash drive.)
If the computer does not have a built in floppy drive, the second method at mickliq's reference is the easiest solution - then you don't need to install SATA controller drivers in order for the XP CD to find the SATA hard drive. You can install the SATA controller drivers after Setup has finished, then change the bios setting for the SATA controllers back to SATA or AHCI mode.
"will it uninstall xp this way? My teacher downgraded laptop frfom a vista to a xp never deleted VISTA so now she wants to uninstall it. So it is a dual boot on the c drive (vista and xp)."
When you boot the computer from the XP CD, the dual boot choice will not appear.
You delete the existing partition the buggered up XP is installed on, then make a new partition in the un-allocated space made available by doing that,then you continue on and run Setup.
If you want to get rid of the Vista partition and all data that was on it too, delete all partitions before you make at least one new one, and then you run XP's Setup.
When you have only one hard drive...
Windows Setup defaults to making only one partition on a hard drive (or, a brand name software installation usually has only one visible - in Windows itself - partition on the single hard drive) .
The problem with that is if you ever need to re-load Windows (or the original brand name software installation) from scratch, you lose everything on the partition Windows was installed on, and when you have only one (visible) partition on the hard drive, that's everything on the drive - unless you copy the data you don't want to lose to elsewhere BEFORE you install Windows from scratch (most people don't bother, and lose all their data) .
If you're installing XP from a regular CD, it's recommended you make at least TWO partitions on the drive.
How to make more than one partition on a hard drive, when you're installing Windows on a blank hard drive, or when you are deleting the existing partition(s) on a hard drive before you run Setup .....
See Response 3: