XP Pro Reboots

Thanks squirrelnmoose.
Just to let u know it was a clients comp that I have now given back, but I still need to go back and fix this current issue.
When I received the comp I was told that it won't log on and when it did that it was slow, but the strange thing with it was when you went to log on it would ask to activate windows before you could go to the desktop if u refused it would kick you back to the log on screen, if you accepted then it would come up with the blue let activate windows now screen, then it would tell you that it had been activated with the product ID to many times so you would have to ring Microsoft and go through the activation procedure, that was all fine until you did so many restarts etc.. after 24hrs I had to go through all of the above activation procedure again, the client said that they had to do the same thing as well, after I did all of the above scans mention before and all windows updates we now have no more need to activate windows (touch wood) but have this rebooting loop if you don't log on with in about 1min of the log on screen appearing, if you walk away for 5-10mins and come back to find the loop still going so you just wait until it loops back to the log on screen and then log on before you miss it again once you are logged in then all is fine.So you can understand why I couldn't do a system restore because if I did then maybe I would have to activate windows again and now that I have done all the scans and updates I don't wont to do them again either, so my only three options are :1 Fix the current issue :2 Let the client put up with it : 3 Re-install windows XP Pro.
Thanks
AJ
Regards.