Okay, now we're getting geeky.  I ran into this situation this week and figured, if it had happened to me, it might just happen to you too.

I had a client who's Windows XP install had become very unstable and we needed to do a system restore from an earlier date.  Problem is, the computer would not boot into safe mode.  So, after some looking I found the following:

If you can't boot normally or in Safe mode, You can boot to Safe Mode with Command Prompt. NOTE: You can NOT start the System Restore tool from the Recovery Console.

After booting to Safe Mode with Command Prompt:

1. Log on with administrative privileges. 
2. Type: %systemroot%\system32\restore\rstrui.exe and press Enter. 
3. Follow the on-screen instructions.

Voila!