When you start Windows, you receive an error similar to:
File is missing or corrupt: C:\Winnt\System32\Drivers\Pci.sys
Pci.sys is a Microsoft Windows PCI driver located in the C:\Windows\System32\drivers directory and is part of the Windows operating system and needed by Windows to run properly.
When this happens you should restore the file using the Recovery Console or perform a repair installation of Windows. The Recovery Console is used to recover or repair Windows when your computer does not start correctly or at all. The Recovery Console gives you limited access to the NTFS file system, FAT, and FAT32 volumes. It's a command line interface that requires you to know the needed commands in order to repair your installation of Windows. A repair installation of Windows will reinstall Windows over itself and leave your programs and files intact. This only applies to older versions of Windows like XP and 2000.