Run a system file check. If any of the files are missing or corrupted, the Window7 DVD will be needed. If you haven't got a 7 DVD, there might be an image of it on the hard drive, and you might be able to use that.
How to use the System File Checker tool to troubleshoot missing or corrupted system files on Windows Vista or on Windows 7
If that doesn't help, run a disk check
Using Windows 7, how do I run CHKDSK? - Microsoft Answers