Log On - Shut Down

I'm not sure if this is the right forum to post in or not ??

If it isn't I apologize in advance ...

I'm emailing for  friend because he no longer can boot up and get help on his own.

he has a dimension 2400 series w/ 512 mb of ram that was working fine up until this morning.

Running windows XP home - SP 3

The problem is this ....

The computer boots up fine to the welcome page where he has set up multiple account for various family members.

when clicking on any of the accounts ... it briefly says .. loading personal settings ...

then logs off - back to the multiple accounts page ??

Any ideas, suggestions on why all of the sudden this is happening ??

Any "Fix It" / "Try This" ideas ??

Thanks in advance for your help

Scott