Sometimes when you switch on your computer an error is being displayed on the screen like.
File is missing or corrupt: C:\Winnt\System32\Drivers\Pci.sys

Whenever you see such error you should directly restore the file using recovery console or insert the windows bootable CD and perform the repair option of the Windows. To conduct the restore installation start the recovery console and select which installation you wish to access and type the admins password and after that type as MAP which will automatically decide which drive letter is allocated to CD drive and type as
Expand <CD-ROM DRIVE:>\i386\pci.sy_ c:\Winnt\System32\drivers /y. and leave a message over their indicating as pci.sys and after that type exit and restart you computer.

Kindly note that c:\winnt should be replaced with the location of your Windows installation.