cpu - Heat Transfer Paste reapplication

I am assembling this PC http://www.codinghorror.com/blog/2007/07/building-a-pc-part-i.html

Of course, I'm in Geek Heaven. :D

I installed the heat sink fan in the wrong direction, and now I want to remove and reattach it.

It came off easily enough.

Now do I remove the old paste from the top of the microprocessor and the bottom of the heat sink or do I just put additional paste and that's it?

If this is a new paste you just put, you only want it to be a uniform thin layer on top of the CPU or the heatsink (don't put it on both surfaces), so you don't need to scrape it all off completely. I usually use an old credit card as a spatula to spread the thermal paste thinly and evenly. You can use a similar soft plastic tool (you don't want to damage either the CPU or the heatsink).

If this is an old paste, it is better to scrape it off and put a new one if you removed the heatsink from the CPU.

If you managed to make a big mess, it is sometimes easier to just scrape it off and re-apply. Some pastes contain silver particles, therefore it is slightly conductive. You don't want that paste on anything else except the surfaces it is intended to be as it might create a short circuit.

Update: According to Intel's instructions on how to apply thermal paste, soft, dry cloth or tissue should be used to wipe off previously applied/used thermal paste. My opinion is Isopropyl alcohol, Googone, or other substances are risky because they can either contain too much water or be too chemically aggressive. Soft, lint-free cloth works just fine.

Remove the old paste before you do anything. You can use rubbing alcohol (denatured iso-propanol). Try not to use Acetone (nail-polish remover) as it can damage the PCB in case of a spill. I'd suggest applying a rice grain size and letting the HSF do the work. The purpose of the TIM is to fill in the gaps, not act as a barrier between the surfaces.