CPU or motherboard is fried. How can I tell?

Hi. I ran speedfan utility on my home-built system. It showed that my system temp was 124 degrees celsius. I didn't have a fan plugged into the sys. fan connection built into the motherboard. I had recently reset the heatsink/fan combo on the CPU, but didn't completely remove all existing thermal paste. I have since learned that was wrong. That's why I don't know if it the CPU or the board itself. Windows XP runs extremely slow. I tried to back up my docs., music and such, but it ran too slow and finally failed the burn three times. Anyone know how I can better ID the problem? Additionally, the OS has not allowed security updates in about two months. So at this point I'm not sure if it is the CPU, motherboard or a bad install of the operating system that has gone bad. Please help. Thanks.