If i have pre-applied paste on my fan, should I clean it off and put on a new one if I want to intsall a new processor?
Dont!! if you do not plan on Overclocking you cpu (In which case you would anyway need a good aftermarket cooler), please do not remove the pre-applied thermal compound. It is a good quality paste and there is a big misconception about it's quality and the notion that it needs to be removed.
That information may be a bit dated but it is non the less accurate. the other compounds in that round up (which are recommended as replacements) have been around for quiet some time as well so that review is totally valid today as well.