I seem to remember!!!!
Now this is a few years ago now there is a reg key which controls it.
I have had it stuck and the problem was a patch was failing to remove the key.
Have a look at this.
Disable Automatic Reboot after Windows Update | AskStudent
Its not really curing the problem though.
I have a feeling it was another key I tampered with. Unfortunately I don't have access to the MS knowledge bases any more which was where i found it the first time.
But just found a link....
But you all have to promise not to tell Loose Rivets about it...
TechNet Library Home
If you search for xp reboot there are a heap of articles.
seems its a well known issue with SP3 you need to reboot using a CD and replace a file.
WIndows XP Service Pack 3 (RTW) Reboot Loop! NO BSOD OR GDI ERROR