Ive had a similar problem after reinstalling XP and installing SP2. Come to find out that there are a multitude of problems that can cause the rebooting. These are the most common:

1. If you have an Nvidia card (GFX or Tuner) its driver is prolly having a conflict with DEP.
2. On some occasions installing SP2 on an "unclean" system can cause problems.
3. DEP and AMD64 processors also conflict with each other.

in the case of problem 1, just update to the newest drivers you can find. Nvidia claims they have fixed the conflict with DEP.

in the case of number 2, try reinstalling xp and sp2 on a clean drive( this means new or formatted. and not a quick format either) install xp then sp2 then chipset, ethernet, modem, sound, video. If this still doesnt fix it...you may want to read up on slipstreaming sp2 onto a XP cd and try that way..thats what i did to fix my constant rebooting.

and for problem 3..theres really no fix..its in MS's hands for that.. but you can always disable DEP and see if that works. which is what i would try suggesting before any formatting, reinstalling, or slipstreaming. Hope this may help.