Computer Randomly Freezes

[crystal2005]

I have a Emachines T1100. The bearings on the fan were wearing down plus fan/heatsink very dirty. I ordered new unit from pcfactoryoutlet.com I installed the new heatsink & fan. Started computer up, fan running fine, but computer froze before windows would even load. Finally after several attempts, I was able to load windows xp however it will run anywhere from 30 seconds to 15 minutes and then freeze up. I have ran all necessary virus/s[yware scans which is not the problem. I have ran the computer in safe mode and it still freezes up. I have reinstalled the old fan, and then put the new fan back in again to make sure it is installed correctly and it still freezes. I know it must be related to the installation of this fan since the computer was fine before this happened. Any suggestions here? ( I have also installed a temperature monitoring software, but it is not overheating.

[ktkendall]

Did you clean off the old thermal pad and apply fresh thermal compound to the new hetsink??

[Marke522]

Make sure you not putting the heat sink on backwards, it should fit only one way, but sometimes there are models that look like the are reversable, they are not. It won't have complete contact with the processor if it's not on right.