Well, I'm having the same problem with my home pc.
I also built my unit from components, so I have no tech support people to harass. (and no, I'm not going to even bother calling Microsoft).
My problem differs slightly, in that I haven't determined what action(s) cause the random reboot. One day I was just listening to mp3s and it rebooted... other times just browsing the web.
Unfortunately, I haven't had the time to isolate the problem... but I suspect it is one of 3 things:
1) Bad RAM -- I have 512MB of DDR, which I tested using software apps -- although I understand software tests aren't the most reliable.
2) CPU overheating - According to the BIOS, the CPU core temp is within normal operating temps -- but that's without XP running.. so who knows
3) Corrupt OS -- I don't really think this is.... but a guru at a local computer parts store suggested this. I'm going to upgrade to the Pro version later this week... and am going to do a clean install to isolate this issue.
Then again.. XP could just be crap... LOL -- but it sure does look pretty
Good Luck!
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Eric Happel
WebsiteBilling.com