Can't load Windows XP

Using winxp pro sp2, intel 2800, 2meg ram,80 gig free hard drive. Yesterday

auto update tried to install sp3 and failed twice so I canceled it. Used the

computer for several more hours with no problem.

Turned it on a few hours ago and it loads as far as the sign in screen -

choice of me or administrator. It always loaded directly into window since I

never set up a password - don't need one.

No matter if I hit admin or myself, it says loading settings, and about one

second later, it says it's logging off and saving my settings. It then goes

back to the sing in screen again. I also tried guest account.

I tried the F8 and every mode listed, but it still insists on a passwork. I

did try every password I've ever used for anything, but no good.

Any idea how to get past this thing. I need to get work done. Thanks