OK, I know this is a little old... but since I have just been through the same thing.. I figured I may as well post what I *think* fixed my problem.
x2 4600+ - stock cooler
2x seagate barracuda sata 320gb (no raid, nothing, one disk split into 2 partitions)
2x pioneer opticals
win xp pro sp2
drivers like dual core optimizer etc installed
anyway, I had the hangs and reboots in the OS, mostly when running bbc climate change program. I also had the startup failing to recognise that I had 2 hdd, and if this was the case, it would normally hang at the windows xp loadup. Flashed the bios with f5, and it still didn't help much. Swapping gfx back to a x850 made the problems occur even more.
On one loadup, I noticed that Cool and Quiet wasn't working, and wondered why. I had also noticed that I thought the cpu fan was spinning very slowly, at 800ish at idle. Anyway, found that the power options were set to desktop, and set it back to minimal power management, for CnQ to do its work.
As I was going back to My Computer (right click, general, to view the clock speed, to see if it was running) I immediately suffered another reboot. Suspicious, I turned CnQ off in the bios, and while I have only had the pc running a couple of hours now, all the problems I was having have gone... well... touch wood
Just thought others might be helped by this... obviously I haven't bothered to re-enable CnQ, as the cpu fan now spins faster, around 1500 at idle, and a better 2400 at load. Meh... it seems to work for me
My system temp is 25, cpu 45, under 50% load... so pretty much dandy...
edit: one issue I still have is that the second gigabit port still won't load its drivers properly... but compared to the [content edited] I have been through the last couple of days... I'll live with it for the time being
heh... I can't even type h e l l..... meh