The only thing that resembles rockbox that will run in Windows is the simulator. There are some ready-built simulators
here that you can play with if you want, though they do have limitations (mainly, any music you wish to play with it has to be contained in the simdisk directory, since it can't access anything outside that directory). Also, the simulators don't have any mouse controls (except the Cowon D2, which is a touchscreen target and simulates touching the screen with mouse clicks), so you'll have to learn the keymap for whichever simulator you decide to use.
In the end, though, the simulators are not the very best media players, they're really only good to test out code changes (to some extent) and for people making themes...