32 bit versions of Windows can only see 4 GB. Somewhere on Apple's site you can find out which versions of Windows can be installed on whatever version of OSX you're using. Mavericks requires 64 bit Win 7.
edit: Ooops, maybe you were talking about disk space, not RAM. Well, Windows does use up space, and it also (by default) makes a fixed size swap file the same size as your RAM. You can turn that off in control panel, memory management somewhere or other if you think you have enough RAM.