Hello, for XP or Vista try using SFC
Please run System File Checker sfc /scannow
... For more information on this tool see How To Use Sfc.exe To Repair System Files
NOTE for Vista users..The command needs to be run from an elevated Command Prompt.
Click Start, type cmd
into the Start/Search box,
in the list above and select 'Run as Administrator
You will need your operating system CD handy.
Open Windows Task Manager....by pressing CTRL+SHIFT+ESC
Then click File
.. then New Task(Run)
In the box that opens type sfc /scannow
......There is a space between c and /
Let it run and insert the XP CD when asked.