[my shadow]

Hi I am hoping someone here can help me to stop my computer from rebooting on it own; it's driving me nuts.
First I guess you will need some history:
I have a Dell 4100 Pentium III. Yesterday we installed a new 60g hard drive and a full upgrade version of XP Home Edition. ( We had winME ) We also upgraded the memory to 512. We have cable internet service with a Netgear Router that has a build in firewall.
I spent most of the day reloading everything and getting it set up. One of the first things I did was to reinstall the Norton's anti virus and do an update. So I really do not think I have any kind of virus, although I did do a full virus scan.
I have found my way around the XP pretty well and did some tweaking, nothing fancy. I have spent hours trying to figure out why this rebooting thing keeps happening. Exactly every 15 mins it reboots. I am thinking there is a setting that needs to be clicked or unclick, I just can't find anything that seems to work. I have read dozens of post here and tried a couple things that sounded as if it may work but no luck. One of the things was to turn off the hibernation, and the automatic reboot. No luck.
I really hate the thought of having to haul this thing to the shop just to have them make some strange setting that I could have done myself.
Thanks for any help that you may have to offer.