On a related note, I also keep an old pc around as a learning box. It's great to be able to easily wipe the hard drive, install a fresh *nix and have a development environment set up that keeps me fearless.
Where I might be concerned with muddying my main pc, I don't feel any problems with doing something potentially hacky or dangerous on the old box. Freeing up this mental space makes the first few steps of learning some new language or environment much easier for me.
It goes in a 5.25" bay and allows you to easily swap out 3.5" SATA drives. You could then have two SATA drives, one drive is your normal OS, and the other drive would have your muddying OS.
There is also virtualization. Not sure if that is an option for you.
The advantage of a second box is that you can use the main and test machines side by side. if you switch drives, you're cut off from your main development environment while you're using the other system.
There are disadvantages too, of course, such as increased desk space, power usage, and heat in your office, and inability to work on the subway.
VMs are good, though VMs don't solve the original problem of letting you test on an older CPU. (Their IO is generally slower, but not in the same way as an older machine.)
Where I might be concerned with muddying my main pc, I don't feel any problems with doing something potentially hacky or dangerous on the old box. Freeing up this mental space makes the first few steps of learning some new language or environment much easier for me.