Random reboots

I've been having this problem for a few years now. I get no warning message or anything. My pc reboots like someone hit the reset button. I get an error message on startup saying my pc wasn't shut down correctly but that is it. It tends to happen more when playing games but I've had it happen when I was just surfing the web. I first built this PC and started using windows vista, I thought that was the problem so I downgraded to XP, same issues. My buddy and I built the same PC's at the same time so he lent me his desktop to swap out all my parts one by one to find the problem, nothing worked. Finally I stumbled upon an article on irq conflicts and shifted my XP pc from an MPCI to a standard PC and it fixed. Now I upgraded to windows 7 and the random reboots are back. If anyone has any suggestions, please let me know. Thanks in advance.

voltage issues and overheating can both cause a PC to just shutdown/reboot. I would check the temperatures on your cpu and video card first. Make sure you fans are working and that the inside of your case is reasonably clean/dust free

Yeah, I've checked fans for everything, reapplied heatsink for the processor, installed a higher wattage power supply, swapped every component inside the box for my PC back when I was using XP and it still rebooted. Even had some programs to monitor voltages and temps the whole time I was running. It's very random. I can run 2 world of warcraft clients at the same time for hours. Then later in the day, just go to use one and it'll reboot within a few minutes. Sometimes I go days without any reboots. I think it's an IRQ conflict but I only found the one way in XP to shift IRQ's and I haven't been able to do it in windows 7.