Why is my Macbook Pro CPU usage so high?

I have a Macbook Pro I got in 2009. It has 2.26 GHz and 4GB of memory, but the system usage is always so high. If I'm running Chrome, iTunes, Last.Fm Scrobbler, and Skype all at once it says System and User is using a total of 100% combined and has 0% idle. It always freezes and lags and never runs smooth. Why is this? How can I fix this? Please help!!!!