Leopard Freeze and slow boot/shutdown

I have had leopard installed since it came out and running a macbook 13". Last week my computer started to act up. It freezes for a couple seconds and then it unfreezes. Sometimes ill put my mouse over the dock and it will take 20 seconds for the dock to appear and it looks like it struggles to show up. When i am in firefox and i quit out it will take 20 seconds or so to close out. Many times when i go to turn it off and click the top menu it will take a while for the list to pop up. I really dont know whats going on. I tried disk utility and it said that there were a bunch of changes to files however it could not repair them. Do i need to reinstall?