****** BEFORE re-assembly
1- clean material off of cpu (careful!)
-- use a tool with a dull edge to scrape away the thermal conductive paste on the processor and the heat sink (the processor is the chip under the heat sink, the heat sink is the x shaped assembly that is attached to the fan

2. apply a fresh coat of NEW thermal paste to the processor, then re-mount the heat sink
-- google "applying thermal paste to CPU" for hints and tips about the right amount of paste to use and the correct ways to apply it.