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.