Go Start>Run ("Start Search" in Vista), type in:
sfc /scannowClick OK (hit Enter in Vista).
Have Windows CD/DVD handy.
If
System File Checker (sfc) will find any errors, it may ask you for the CD/DVD.
If
sfc won't find any errors in Windows XP, it'll simply quit, without any message.
In Vista you will receive the following message: "Windows resource protection did not find any integrity violations".
Navigate to
C:\Windows\Logs\CBS folder. You'll see
CBS.log file.
Usually, it's pretty big file, so upload it here:
http://www.uploadbigfiles.net/, and post download link.