At some point you surrender! I don't know why my computer gets slow and cranky from time to time, would expect 'memory leakage' whereby some programs leave bits of used memory even when the program is shut down.
I stuffed this PC with 4GB of memory, though, and that should be IT!
The cure is to reboot from time to time (and generally I leave it on until I have to reboot, so it can be days or weeks - usually days.)
Ah, but there's some process running in the background, you say. You nerd. I knew that. Some I even want to run. Others I don't know have come to live in the start tray. (Yes, I know how to disable that/them.)
Generally the computer works very well. Until it doesn't.
You've been there, right?