Mac OSX works differently than what Windows does when it comes to closing windows and programs. Mac users that come from windows typically are leaving programs running and do not even know it. If there are a lot of programs then your Mac could start running slow because all of the programs are using up memory.
I found this out first hand recently when visiting some family members. This episode of Mac Minute show you how to close programs when you are done with them so they are not using up memory.