New
#11
Replacing a CPU with one of the same socket type (has to be the same) is fairly straight forward. Take out the old one, put in the new one and lock the heatsink onto it. This is assuming ofcourse that there is thermal paste on the new CPU already (last cpu i bought already had thermal paste on it).
As your using a P4, your mobo can only support single core processors so you wont be able to really upgrade.... You would need to buy a new mobo, by doing that you need a new CPU (as the socket type on the new mobo will be different to the P4 socket type) and while your at it may aswell chuck in some new ram. In the end you pretty much have a new computer.