Can you trace back to when you first noticed it slowing down, and if so, did you change anything or install anything, no matter how small?
Also, if you're on Tiger then download 10.4.9 from software updater. It's a good update regardless, but it may sort your issue in some way.
When you have time to mess about with it, try restarting then not running any apps and see if it's slow after an hour still. Then try running your apps one by one, to see if you can narrow it down. From the CPU numbers though it sounds like it may be Safari, but really it's a little low on the percentages for it to be that noticible.