A while ago i was playing agame and i heard a pop or bang insied my computer. There was a broken plastic bit. My computer would freeze after 10 minutes then i cleaned of a lot fo dust from the cpu fan and now it crashes when i burn a cd, play games, certain websites and othr things. WIht a loud noise coming from around the cpu area which gets louder until it eventually ffreezes. The picture here is not mind but it shows the plastic part thats snapped where the red arrow is. And it looks liek sumthin may be broke in the right hand corner but im not to sure. Does this mean i have to replce the heatsink and fan, will i have to use thermal paste.

I have a p4 2ghz

Sounds like it is probably overheating as the sink is not making good contact. Get a HSF amd yes use a small amount of thermal paste.