One of the annoying things with ALL Windows OSes is that sometimes, drivers don't get installed properly or driver conflicts arise in the damndest ways. On my old dual-core XP system, when I got my new G110 gaming keyboard, the first time I installed the software there were no problems save for a tiny technical issue, so I installed the latest drivers instead. When I did this, one CPU core would suddenly max out constantly at low priority, so I could still use my computer fine, but at idle I was seeing a constant 50% CPU usage. I tried reinstalling the old drivers but oddly had the same problem.

Eventually, I narrowed it down to a single device driver fragment in the device manager which was now being installed redundantly every time I tried reinstalling the software. Disabling the redundant device solved the problem but I still never found out why that was happening, let alone why the very first time I installed the drivers I didn't have this problem. :P

Your problems are likely something like that, where the drivers just aren't cooperating because of other things you wouldn't even think were an issue. I doubt it's a power-related thing if your fresh install of XP had no issues at all.

Does XP still do that thing where it will completely disable DMA on an optical drive the first time it has trouble trying to read a CD? Perhaps this problem is related. See for instance http://www.onthegosoft.com/dma_setting_nt.htm .

Is there any reason why you don't want to just do a fresh install of XP and then re-install your software? It sounds like you've already done the equivalent amount of work during troubleshooting. If you want to try one last measure, unhide your device drivers via this guide and delete any leftovers from your old motherboard and CPU, then install Service Pack 3. I did this after a motherboard swap and was able to remove many phantom devices.

Also check to ensure that your optical drive is configured properly (Master/Slave for PATA). I can almost guarantee that it's not a power-related problem, sounds much more like a low-level system configuration / device driver issue.

This is a major pain in the xxx if it happens, and whenever it happens, since anything under the sun can cause this .... pain.

It can be anything from an installed software, to drivers to connected peripherals - USB peripherals specifically. When XP expects a hardware to be present after the drivers are installed (usually those drivers installed by Windows itself during installation automatically) and when these hardwares are missing, it can happen too.

Only last week I faced this problem in my XP in my notebook - unable to shutdown or reboot - it just stays there in the desktop, still working as if nothing happened. But sleep worked without problems. I found out that the No-IP DDNS client software caused this issue, and it went away after I exit the program.

When Windows works, everything works well, and whenever something goes wrong, it makes you want to pull your hairs out.