Rockbox relies on the presence of several parts of the original firmware in order to boot. The iPod Video cannot boot at all unless the Apple firmware partition is present and contains several important boot images (including the firmware for the onboard video chip). Even if you don't use iTunes, you still need to create this partition and copy the Apple firmware into it before it's possible to install Rockbox, so unless you have absolutely no way to get access to iTunes on Windows, it's much easier to just use it.