Hmm.. i think the P3 should be just be able to run XP... make sure you are disconnected from the network and/or internet...

Are you able to boot directly from the XP CD? The problem could be caused from trying to boot to a DOS prompt using the WIN98 floppy.

Also, check all the cables from CD and HD to motherboard. Finally, check BIOS settings, and revert to fail-safe if need be.