Restart your computer in Safe Mode:
Click Start
Type: CMD, from the results, right click CMD
Click 'Run as Administrator'
At the Command Prompt, type: sfc/scannow
This will check for any integrity violations
Restart your system
Also check your hard disk for errors:
Click Start
Type: CMD, from the results, right click CMD
Click 'Run as Administrator'
At the Command Prompt, type: chkdsk /r
When you restart your system, your computer will be scanned for errors and attempts will be made to correct them.
Also, if that does not work:
1. Insert the Windows 7 DVD
2. Restart your computer
3. When asked if you want to boot from your DVD drive, do so.
4. Choose your language, click Next.
5. Click 'Repair your computer'
6. Select operating system you want repair.