Try a repair of your operating system?
Vista or W7 use the DVD or tap F8 while booting and select repair from the menu.
Repair XP by replacing corrupt files
in the run box, press enter and let system file checker find and replace missing / corrupt file in windows and explorer.This will take awhile to complete and you may be asked for the CD to be in the drive if windows is not preinstalled on the hard drive.
sfc /scannow problems