One of the fun and wonderful thing about computer problems is trying to figure out whether they are hardware or software related. Since it looks like your main hardware (at least that which can easily be tested, and leaving the PSU as an option to consider later) is all right, let's jump back to the software side of things.
Try running msconfig and disable everything on the "Startup" tab. (You might lose some features of some of your hardware, for instance if you have a fancy-pants keyboard with lots of extra buttons those extra buttons may not work.) This will at least eliminate a few of the "always running" side programs which could be causing a problem.
How often are the freezes occurring at this point? Ironically enough, the ones that are few and far between are harder to track down than the constantly annoying ones. With the latter you can at least tell right away if something you've done has helped.
PS: If your wife comes at you with a meat cleaver, a heavy serving tray works well as a shield. (Don't ask me how I know that.)