It sounds to me like your cpu was running so hot that it melted the thermal paste and then caused the whole thing to overheat even more until you had problems. Sometimes, along with the cpu running hot and the thermal paste melting or losing its power, the computer can get banged around so that the hs/fan gets moved off-center and that causes more over-heating probs.
Many times i have had people bring their puter in for repairs and i have seen the hs/fan not on correct, maybe it got pushed or moved and they never noticed. I have seen too much dust and dirt inside the heatsink. I have seen heatsinks full of drywall powder, i have seen almost everything there is to see. In most cases, its a matter of the heatsink/fan either not being installed properly in the first place, or poor thermal paste, or the heatsink/fan was moved a bit and not making good contact, and maybe the thermal paste drying up so there is little contact. So many things it could be.
To jinsonxu and Lutrosis
Presuming you are both in win xp, then disable automatic reboot as redbeard suggested and also make a log file as i suggested earlier on.
Then post results here and we can help you out.
That is, provided that your heatsink/fan is on correctly!