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Any Mac Mini designated as "new" since 2006 (with at least 4 USB ports on rear) can have Windows installed on it. Apple's Boot Camp offers its own partition formatting for dual boot into Mac OS X and Windows which you can later wipe out the Mac formatted partition if you so desire. However, before thinking about doing that, you may be surprised to learn most applications which only run in Windows have Mac compatible counterparts*:
http://www.macmaps.com/crossplatform.html
Not to mention, the Mac Mini doesn't have the strongest graphics card for games. If your desire is for that, you may want to research what people have done with Boot Camp here:
http://discussions.apple.com/forum.jspa?forumID=687
and discover which Mac meets your needs.
- * Links to my pages may give me compensation.