Any good PC diagnostic software out there?

Calling out to the Tribe... can you recommend a good hardware troubleshooting program for a desktop? Either running in Windows, or bootable off a disc/USB key is fine. Either that, or read the symptoms below and offer a suggestion as to where the problem lies...

Here's the scoop, I've got an older Core i5-650 with 8GB of RAM and a 1TB hard drive, and a GTX 460 graphics card as my home desktop, currently running Windows 8 Pro.

Late last week, it started freezing up on me, but not completely.

Symptoms:

After between 5-10 minutes of use, the system enters a weird state, where the mouse cursor still moves around, but new windows won't open.

If I walk away from it, the monitor will (sometimes) wake up when I hit the spacebar or wiggle the mouse, but it's still in the same state.

I've tried swapping drives with a previous backup. I also tried a fresh install of Windows 8. It even exhibits the same problem if I boot from my WHS recovery disc, so it's not Windows 8 specific.

I've never seen this specific problem before, in 20 years of building my own systems for both home and work, so it's a PITA to diagnose properly. If anyone has seen this specific problem, and can point me in the right direction, or a good tool to run diagnostics, that would be great.