I've had a bit of a search for the same problem, and as Unne said, there aren't any helpful hits. The reason for that is that there seem to be 150 different fixes that may but probably won't work.
One person seems to think flashing his BIOS fixed it, while reinstalling video drivers worked for someone else, etc.

A simple thing you could try would be to browse the XP CD on your Mac, try to find the AGP440.sys file, boot into Knoppix from the live CD and copy the file over using a flash drive or something similar, though that actually probably requires some form of decompression utility to extract the file.

You could even try stealing the file from a library (or other publicly accessible) computer!

Quote Originally Posted by Omecle
Hmm, I'm not quite sure if I understand that. Doesn't the upgrade to Windows XP turn the drive format to FAT?
The upgrade likely would have formatted the drive as NTFS, rather than FAT32. FAT32 was used by Windows 98, mainly.

From a Knoppix CD, mount the drive and then run "fsck" from the command line; that will check the filesystem for errors.