|A better description of the problem would help!|
Are you getting a BSOD which says system32\DRIVERS\pci.sys is missing or corrupted?
If so, something like a BartPE disk would be useful - or another machine to mount the drive in. Because the first thing I'd want to do is run chkdsk (can also be done from recovery console). Then try booting - if no good check to see if pci.sys is really there or not - copy new one from good installation, and try booting again. If still no good, try a repair reinstall - you'll need XP install disk from your version of XP.