Hi wOlverine,

1. Replacing the fan isn't the only thing, it must have a suitable rating for the processor.
2. The heatsink is generally more important in day to day use.............is it approved for the processor?
3. You need to look at the interface between the heatsink base and the surface of the processor............is the heatsink firmly seated? is there any white stuff oozing out? You DID use thermal compound I hope............and NOT the white stuff?..........OK the silver compound is expensive.............so is a new computer, and the silver compound is at least 10x more efficient.
4. You should have an exhaust fan, these fit into a spare ISA/PCI slot and pump stale hot air out of the box at 40-60 cubic feet a minute.
5. What is the temperature cut off set at in the bios?....I am assuming you have that facility?

What makes you so sure that it is the CPU overheating?.............if it is your machine shutting down, that could also be a virus...........particularly if you are in India, there has been one going round with a timer of about 15 minutes on it, in that part of the world.

Please give us some feedback

Cheers