Looking at it, it appears to boot correctly to the log in screen, we have 2 user names, on selecting either of them it reports logging on (after 2-3 secs) it gives the logging off sound followed by the logging off screen. I have tried to get past this in safe mode but it still does the same with the addition of Admin user. I then decided to do a repair on XP, again after the repair it still has the same fault.
I suspect a virus, can anyone confirm this and offer any advice as to how to delete this. I know as a last resort, it is a reformat and fresh install.