I'm running XP Pro on an Athlon processor (not an XP processor; an older model). The pc started rebooting all by itself, sometimes after running for a while, sometimes not even getting past the startup screen. Since I thought it might be a heating/cooling problem, I removed the hsf, cleaned it, remounted it (using thermal compound), and now it won't start at all. The fan starts up, the light on the dvd drive comes on, the hdd light comes on (sometimes), but no display on the monitor, no beeps, and the hdd light goes back off after a little bit. I tried reseating the RAM, no fix. I can't imagine that it's my hdd, since it was working fine before I replaced the hsf. Everything appears to be plugged in correctly; the fan is mounted in the right orientation (it's a socket a, and the ridge is aligned with the raised part of the socket). I'm at a loss as to what to do next.
In case it matters, it used to be that when I'd turn off the pc with the power button, unplug and replug the power cable, the fan would start up for a few seconds and then stop until I actually hit the power button again. Now, following the exact same steps, once I replug the power cable the whole thing comes on. Is it a power supply problem? It's a three-year old Compaq 5106cl, 250w power supply.