why is my computer so slow?

Since some days ago, my mac is running extremely slow. It takes a long time to open a page, a new window, to surf the net, etc. I initially thought it could have been related to something I downloaded using bittorent and therefore though it was a virus/malaware. But then read that having a slow computer is not a sympthom of malaware (source: http://www.reedcorner.net/mmg-infected/).

So then, my question is: why is my mac sooo slow and how can I get it fixed? (I have a macbook pro, os x 10.6.8)

Any help is highly appreciated!