Does it pop up with anything about activation? If you refuse to let it activate windows, you get logged out immediately. If it doesn't, something is very wrong. Maybe a virus of some sort. See if you can log in using safe mode (F8 during startup) and try to run a virus scan.
Go to this website if you have an Userinit problem, although there are many other variations to this error so it might not be the Userinit.
http://www.winxptutor.com/wsaremove.htm
Start at the header called "Enter the Recovery Console" then go to Phase II and so forth.
Maybe you should try to do a System Restore to an earlier configuration?
Although I have read about this online, I haven't been able to find a solution using Safe Mode..
Apologies..
If all fails with Safe Mode, I recommend you to this website http://www.logicnest.com/archives/90 or http://support.microsoft.com/kb/555648 and use one of the (different) solutions ( Maybe?-> that hasn't been discussed already.)
EDIT:Ah fudge, it talks about the XP disk, but the "Slightly Harder Fix" might do the trick.....
Not sure, where did you see that it needs XP on the "Slightly Harder Fix"..Yeah safe mode didn't do squat...
I read the "slightly harder fix" and it says I need a computer with XP on it. Will Vista work as well? (My parent's computer is the only other computer I have...)