macbook pro fan always on

Hi,

So as of a couple of days ago, whenever I use my macbook, doing some very basic tasks (like using the internet/word processing/whatever) the fan always revs up and works quite hard.

I am aware of that sometimes programs are stuck on high CPU (i.e. flash) and hence take up quite a bit of memory, but these last couple of days, nothing is out of the ordinary when I check the activity monitor and normal amount of memory is used.

So now my computer is constantly very hot and fan is always working hard? Any ideas how I can fix this?

I've attached two images of what the activity monitor usually looks like.

Could the fan have stuff stuck in it? If so, how can I fix it as I have a unibody macbook pro.

Cheers!