Boot the computer in safe mode with command prompt.
Log on to your computer with an administrator account or with an account that has administrator credentials.
Type the following command at a command prompt, and then press ENTER:
Follow the instructions that appear on the screen to restore your computer to an earlier state. (A date before you forced the boot to safe mode)http://support.microsoft.com/kb/304449/
Do you have a Windows XP installation CD?