When the CPU usage is running at 100%, that's the time to see which are the "CPU hogs" - which the task manager will tell you.
My machine ticks over with "System Idle Process" ticking nicely between 95% and 98%. However, to get there (from 65%-ish) I've had to disable one or two greedies, especially iTunes - which is only needed when there's an iPod actually plugged in. MSCONFIG is a very useful utility!
One "fix" I found recently, and which may apply in your case, is that if you're running Firefox v2, it needs to be shut down fairly often. I'm assured there isn't a memory leak in it, but on my machine it will increase memory used over a period of hours, until it's up around 400MB. The PC runs slower and slower at the same time. Once you shut it down (and it needs a minute or two to "unload"), it will free up all that memory and things go faster again - for a while.
I'm now using Firefox 3 Beta: that's not gone above about 80MB regardless of how long it's been running. There are drawbacks to FF3 at the moment (inevitable with Beta software), but I can live with that! Hopefully, they won't reintroduce a memory leak before v3 goes final.