I'd go GPL. One of the Classic Mac communities greatest achievements for users was introducing apps that could completely customize the user-space. Much like X-Windows is fully customizable if you're willing to dig around in Widget sets and what-not, only using user-friendly apps that tinkered with the built-in MacOS GUI code. Since this was a capacity of the Mac GUI and not really much of a hack, this software was by-and-large free. One could delve into the UI settings and alter all of these components by hand, but that would be a real pain. Also as a result of it being a capability of the Mac UI, it would be difficult to claim ownership of the formats, seeing as Apple owned the UI, and the programmers had just made apps that made modifying it simpler, probably another reason these apps are freeware.
Windows doesn't have real built-in UI customization, something that has always annoyed me (Sure I can change the pictures of my icons, cursors, and desktop, but that's boring).
To sum up a long digression, I think that Windows customization programs should be free (Frankly, I think they should be built into to the OS or UI), but I think that good hacks deserve proper recognition. Go GPL to maintain "ownership" of your app, but make sure the source is open for people who have good ideas of their own about UI tweaking, but don't have a code-base to start with yet.
BTW: I can't even use this good idea, because even changing my wallpaper to an image destabilizes my OS
