Can Not Log onto user account in XP

Running XP home, boots up to the user log in (two users). When the icon is clicked to log in, it immediately logs out again. This is the case for both accounts, even in safe mode which ads the admin account. I’ve tried the [windows][break] key combination to try to activate the system properties dialog, but it won’t take. This appears to be caused by a system error that is causing a continuous reboot loop. How do I fix this? Thanks. Doug.