try this..

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 to repair.

On the System Recovery Dialog, click the Windows Memory Diagnostic link to run a scan of your memory modules for errors. If it detects errors, that could possibly why you are experiencing stability issues.

If its not, boot back to the System Recovery environment and use the Repair Installation option.

Also make sure you on have essential peripherals connected, its possible that devices that are not compatible might causing the system to become unstable.