Here are some more findings on Google. I'm quite impressed how many different issues can apparently lead to this kind of trouble, and I guess I'm not through all of them:
- Disable Wake-On-LAN in BIOS
- Look for a possible short circuit - this includes a defective power switch in the case
- Wireless mouse/keyboard (user changed it and reboot stopped)
- Defective soundcard / other PCI card (I guess you tried without it already)
This one sounds interesting:
http://winhlp.com/WxSP2.htm#Boot_or_shutdown_problems_after_Service_Pack_2_installation
The article concerns describes a missing microcode update with i.e. Intel Prescott CPUs and your mobo is on the list of affected boards. You've got a Northwood but check anyway if your BIOS version is 1017 or newer.
So there are some things to check before you take the hassle of doing a clean install on a IDE drive (or at least copying OS from the other).