Usually when I turn my computer on it loads up and goes straight on to the desktop, but now when I turn on the computer it loads up correctly until the desktop is due to load. It then shows the background (of the desktop) with no icons for a few seconds, and then it goes to the log in screen. When I click log on it shows the background of the desktop then goes back to the log on screen, but under the user it says logging off.
It does exactly the same on safe mode.
i am running windows xp professional.

you could try swtching off at plug or holding power buton til it goes off switch off power and then reboot it.

Nope no good i have tryed this. i have also tried to press alt control and delete when the background is up up but again no good.
there has to be a way

when you go into safe mode, have you tried logging in under the Administrator account. If you can get in using this then I would try system restore or run a full virus scan.

i have already tried to enter safe mode but the same thing happens to wen i log in normally

Ok I've removed Silverback's hijack of the thread as that was getting confusing.

Now do you mean that you've tried logging into safe mode as Administrator as Pogue suggested but you still get the same problem so can't try a system restore or run an anti virus scan?

If so I'd reboot and hit F8 again but this time, try picking the option for 'last known good configuration'