If you are unable to start Windows XP, even from Safe mode, and you had upgraded from Windows 98, Windows 98 Second Edition, or Windows Millennium Edition (Me), you can restore your previous version of windows by starting in Safe Mode with Command Prompt:
1. Log on with administrative priviledges.
2. Type C: and press Enter.
3. Type cd \windows\system32 and press Enter.
4. Type osuninst.exe and press Enter.
5. Follow the on-screen instructions.