I installed UltraDefrag and tried to run it. First I tried to do a quick Analyze on an 80GB disk. Within about 20 seconds, wham! Reboot. No BSOD, just a blank screen and then the BIOS screen. After the reboot I tried again, wham! Reboot. Tried once more and made it through the analysis without crashing, so I tried to defrag the volume. Got maybe 5 mins into it, and wham! Another reboot. Sorry, that's enough for me.
Unfortunately I don't think I can offer much additional info. There were no messages, no warning, no nothing. Just an instant reboot. This was on XP Pro SP2.
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