"I've downloaded and installed the Windows 8 Consumer Preview onto my desktop PC and I'm very impressed with it. One thing I wish I could experiment with was the touch interface. The trouble is, I don't have access to a tablet system, and I don't want to buy one yet because I think they'll get better by the time Windows 8 is out for real.
Is there a way I can touch-enable my existing PC without having to throw the whole thing out, and without having to sell my kidney?"
Yes, there is a way, and it's actually quite simple to do. You can replace your existing monitor with a touchscreen panel. And it won't break the bank in the process.
If you're looking for a hardware suggestion, I suggest you take a look at Acer's T231H 23-inch LCD panel. Not only do you get a really nice 1920 x 1080 full-HD screen, but you're adding touchscreen support to an existing system for around $320.
The Acer T231H is a great touchscreen, but there is one irritation you need to know about before you buy. The screen has a bezel around it, and this makes it tricky to get to the edge of the screen to make use of Windows 8's edge gestures.
If you have large fingers this can make getting to the edge tricky. A solution to this problem is to use your graphics card driver to underscan the display slightly to bring the edges in a little. You'll lose a few pixels at the edge, but chances are you won't notice it. Both AMD and NVIDIA offer an underscan--overscan ability in their drivers, so it's quite easy to do.
By the time Windows 8 is released I expect touchscreen monitors to feature a bezel-less design so that edge gestures will no longer present a problem.
If you've got the space and a second output on your graphics card, you could keep the existing monitor on your system and give yourself a dual-screen workspace. You'll be amazed just how much extra productivity you can get from adding a second screen to your system.