It must have a clamp for the cpu I think every PC from 386 upward did. What CPU and Mainboard do you have?

if you have like a really small hs, like one off of a northbridge, then the t paste should be enough. if it is a like a big heatsync and fan. no it isnt enough. so you need to find something to hold it on withl. Find a latch or something that should have come with th eCPU or the mainboard.