you can 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. Follow the onscreen instructions to restore your computer to a previous saved point.