I don't understand your response "nothing open" in Activity Monitor. I have ALL apps closed (quit), except for Activity Monitor, and I have about 45 processes that are open. Only about 6 are active, but that number would be fairly normal, and would increase significantly when other apps are opened.
What did you find out from the system logs in your Console?
I think that you could get some help by using an app that will clean your caches. OnyX is a good one to use. There's lots of other system maintenance utilities. There's a new one, too, called iTweax, which I have been using recently. Shouldn't harm your system, and may get you back to normal (whatever that is...)
Check that you still have sufficient free space on your hard drive - 15-20% free of your total space available should be fine.
Boot to your OS X installer DVD, and run Disk Utility from the menu. Select your hard drive, and click the Repair Disk button (different from Repair Disk Permissions).
Come back with your results...