Do you have a bunch of little programs running, or is there a lot in your taskbar or something? I know that that always tends to add up to something big, even if they are small programs. Are you running a music program? That definately always slows down computers.
I always used to tell people to do that, but have recently got bored of people coming back saying it brings them to another random screen (if they have fast user switch disabled - although common sense would dictate for them to just click task manager)
CTRL + SHFT + ESC brings up task manager. Click processes and order by CPU
If there's an unknown process lookit up, end it if need be and then use msconfig (Run -> MSCONFIG) to disable it from starting if possible.
you can go to run and type in msconfig, and from there in the startup tab, remove some programs that you don't want to run, that way those unwanted programes won't even get initialized!