weird xp logs on and off immediately

Hi guys! I'm still new to the forum but you guys have been really helpful. I was curious about a strange incident which happened today. I started the computer and tried to log on with my account ( There are 3 accounts here ). Xp immediately started loading the desktop- as usual - then it did the most unnexpected thing ever: logged off as soon as it was done displaying the wallpaper. I tried the same thing with the other accounts and it was the same. So, i took an XP installation disk, and re-installed windows in "repair mode" and the problem has gone. That is, I can now log on and type this message to you. I wanted to ask you if this could have been caused by a virus or could something else be responsible for this ?

Thanks
Stefano