You don't need a Mac. You need a USB interface, recording software and a mic. Most of the interfaces (brands like Tascam, M-Audio, Lexicon) are shipped with recording software and sometimes are offered bundled with a mic. I've seen basic packages like this for $200 or less. Just look at the specs and make sure the drivers are compatible, I know that some aren't compatible with 64-bit XP or Vista versions yet.
I plan on getting one when I have the spare cash.