I've a E8400 dual core from intel. I Just removed the heatsink that came allong with the processor for cleaning purpose. Can i just put it back on or do I have to buy new thermal paste and reapply?
If you're not overclocking and have a case fan, I would try reinstalling it. The thermal material will melt once the cpu reaches operating temperature. I've found no more than a 2-3 degree celcius difference over applying new paste. Your idle temp should be in the low to mid 40's.
Yes I have a case fan (antec) and also don't overclock my cpu. Problem is I took it out before thinking of it. I'll have to wait until monday to buy some new one that why I'm considering to just put it back on. But if the thermal paste melts at a normal operating temperature, than I just could reuse it?
I would leave the cpu in the socket. Just apply a small amount of ispropyl alcohol to a clean cloth and gently get what you can off. It doesn't have to be spotless to use the new paste. You can apply one drop in the center of the cpu and press the heatsink on right away, or use a single edge razor blade to even it out on the surface before installing the heatsink. I worked at dell installing boards and cpus; the less you touch components the better. electrostatic discharge can happen any time; it varies with individuals as to the damage. You can use an esd strap on your wrist or shoe if you really want to play it safe. That's why if your temp is in the 40's now, I wouldn't even bother using new paste.