I think iTunes can run on something as old as Windows XP. For your own good, I wouldn't even try installing it on something older than that. The other thing to keep in mind that Apple is not offering the actual program written for XP, but if you had the 64 bit version of XP, it might still be fine.
There used to be a library on itunes.com where you could choose older versions of iTunes. It's not sure if that option still exists. Anyway, when trying to install iTunes, you'll be prompted when your windows version is not compatible.