Thermal paste is fairly easy to re-apply (if you have to), the easiest way is usually to clean the old stuff off with a little nail polish remover (or methylated spirits), making sure you clean both the processor and the heat sink, then apply a thin layer of paste across the processor's heat spreader (I usually use an old credit card or something similar to get it both thin and even) then put the processor back into the motherboard, and reinstall the heatsink. Easy aye