|What was wrong with it that prompted you to use System Restore?|
Do you have an XP Install CD?
If so you should be able to boot up to that and enter recovery console. It will prompt you to enter admin password in order to log in to command prompt. You may be able to start the System Restore interface manually by navigating to %SystemRoot%\system32\restore\rstrui.exe
You may be able to undo the last restore and select another restore point. If your system had some kind of virus or trojan infection then using System Restore will not help as many such infections embed themselves in the restore folder.
Tell the full story regarding your problem otherwise the advice you get here might not be appropriate.
Goin' Fishin' (Some day)