Windows XP Pro SP2 won't load; file corrupted

When I went to turn on my Latitude D620 this evening, XP would not load; I got an error message that system32\DRIVERS\pci.sys is missing or corrupted.  I don't have my Windows XP install disk with me - is there anything I can do?  And why did this happen?  From messages on the Microsoft support boards I gathered this is a problem with Windows Update??? Is this true and if so why can't MS make a reliable product after all these years?

 

Thanks.