What is the CPU type and is it the stock HSF. (HSF = Heat sink fan)
If you took the HSF off of the CPU then you MUST clean the old heat sink compund off and apply new. If this is the case the and you did not clean and reaply the Paste, the CPU will over heat very quickly. You can get some artic silver 5 locally (ie Bestbuy or radioshack).
You can google how to apply the paste and install the HSF, some good videos on the web. (reason I asked cpu and HSF)