Agreed,
You should make a priority of backing up all your important data before
anything critical happens.
Documents, photos, music library, address book, bookmarks, Installer .dmg's and their correspondiing authorization keys, update packages in your packages folder and so on.
It's also a great time to organize all you had stored so it all makes sense.
If you're like many of us, your whole digital life is on that hard drive.
Once you have everything safe, then run the disk utility from your OS disc
first verifying and then repairing your boot drive.
Hopefully your disk will repair without incident.
You should then try running your hardware diagnostic CD to see if anything
shows up.
If you go to this much trouble, I would then consider a clean erase install
only this time running a custom install de-selecting all the extra printer drivers, language files, Software demos, iDVD themes, and GarageBand
demo songs. Doing so will free up 4.7+ GB of space off your hard drive.
A clean erase install is still the best way to insure that all the stuff you have accumulated isn't bogging down your system.
The only other thing that comes to mind at the moment as far as CPU
hogging, is the use of Norton's AV or Virex 7.5.
If you have either of these installed, that could explain why your system is running slow. Both are notorious CPU hogs.
Sometimes a badly fragmented hard drive will cause your hardrive to work too hard locating data.
This is where DiscWarrior comes in handy, both with diagostic tools and defragging.
If the disc chatter persists after a clean install, then you'll know it's a hardware issue.