Use system restore when you cannot boot your system:

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. To do this:
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' and then press enter. Followthe
onscreen instructions to restore your computer to a previous saved point.