Here is a really good video explaining how to fix the missing or corrupted "system32 drivers pci.sys" file properly.

System32 drivers pci sys

Is your system32/drivers/pci.sys missing or corrupt? Do you get an error message when you turn on your computer?

You should try the followig to fix the problem:

Put in your XP-CD-disk and let your computer boot of the disk. You should be able to press "r" to start the recovery-console. Now you have to chosse your win installation.
When you get into the command line, get access to your cdrom-drive by typing :

"D:" ( it could also be E: or F:)

When you have done that type in this:

cd\ <enter>
cd\i386 <enter>
expand pci.sy_ C:\pci.sys <enter>
C: <enter>
cd\ <enter>
copy pci.sys C:\windows\system32\drivers <enter>

Please confirm if it asks to replace the files.
After that you will need to restart your PC.
If there are more missing files you can replace them all doing the same thing again.
Now you should not get any system32 drivers pci.sys error messages anymore.