WinXP starts, says Choose User, and then shuts down!

My brother has a Toshiba laptop w/ a 2.8GHz P4 and 1GB RAM running WinXP. His is the only user account on the computer, so usually Windows boots all the way thru at startup w/o having to select a user account.

But he called me last night and said that now it asks him to select a user account, even his is the only 1 listed. When he selects it, it says something like "saving changes to your personal settings" and shuts down the computer.

I had him try logging into Safe Mode, and the account screen came up w/ his account and an admin account. I told him to try the admin account, but the same thing happened. So I don't know what else to do considering I can't even get the comp started to begin the normal virus-killing procedure. Is there a way to boot into DOS mode, and if so, is there a virus scanner that works in DOS?