It's a system file, so....
Go Start>Run ("Start Search" in Vista), type in:
Click 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.flyupload.com/, and post download link.