System does not boot?

Windows System boot failure:-
If your system has failed to the point where you cannot access the Windows GUI either through booting normally or through safe mode, you may still have the chance to use the System Restore feature if you have it enabled, by running it form the command prompt. ..... Restart your computer and press F8 after the POST screen to bring up the Windows XP boot menu. Choose 'boot in safe mode with command prompt.'

If your system gets to the command prompt successfully, type '%systemroot%\system32\restore\rstrui.exe' [without the quotation]and then press enter. Follow the onscreen instructions to restore your computer to a previous saved point.

If you use Windows vista then you can insert your vista boot dvd and restart your machine then you can see restore option from there you can restore your system into a previous stable point.