No, Windows will not install on any Mac natively because of unsupported hardware. Macs do not use a tradtional BIOS, but open firmware instead, and Windows does not support open firmware, and Microsoft has said that they do not ever plan to support open firmware, hence the need for Bootcamp. I also thought that it wasn't nice of Apple to drop the Bootcamp beta, requiring you to buy the new OS to use it, but it is their software, and they can make that call, just like Microsoft not reducing the price of XP when Vista came out. But that's for a different day. I'm afraid that there isn't much you can do at this point, except buy 10.5, or sell the Mini off to get the last money you need to build your PC.
Oh, if you decide to just find a new home for the Mini, it is always welcome on my desk, and free is in my price range