Denise Griffee's computer freezes up every two or three days. I offer advice.
The obvious quick fix is to not leave your computer on for two or three days at a time. In fact, I'd give you this advice even if you weren't having this problem. Windows tends to get unstable over time, and a reboot refreshes it. Besides, leaving the computer on all the time wastes electricity.
You should either shut it down or hibernate it at the end of every workday. Hibernating the computer won't refresh Windows the way a full boot does, but it's faster and lets you start the day exactly where you left off. From the hardware's point of view (and I suspect you've got a hardware problem), there's no difference between shutting down and hibernating a PC.
But that's hardly a real fix, so let's try to figure out what's going on.
First of all, how exactly does it freeze. If it's with a blue screen filled with white text, see Why Does My PC Keep Dying With a Blue Screen of Death?
Something else to look for: Can you successfully boot up immediately after you've shut down from the freeze. If you have to wait a few minutes--or hours--it's almost certainly a hardware problem. My guess: Either the power supply is dying, or your PC is overheating. I discussed heating issues last year, and you may want to check that article to determine if that's your problem and to figure out a way to fix it. Replacing a power supply is cheap and easy in a desktop PC, although if you have a laptop you should check with your vendor about this.