Logging on Starts to bring up Desktop and then immediately logs off.

[Charles May]

I'm trying to figure out what is causing this but I can't even get in to run
any type of virus scanning software.

When the welcome screen comes up, I try to log in by clicking on the User
and immediately after the welcome screen disappears (doesn't even show the
desktop completely) I get the log off screen.

I have tried Safe Mode, Last Known Good... etc... I can't even run
Anti-Virus to check it.
I even tried other user accounts (Administrator and The Kids Accounts) all
do the same thing.

This is my uncle's machine so I don't know what errors (if any) they may
have received prior to this happening.

This is XP Home

Any Suggestions?
Any help on this would be greatly appreciated

Thanks
Charlie