since you planning for a forum not a sales page or commercial sites, it should be safe to have it, especially since you use the short word "win" instead of the registered "Windows" or "win32" and not using any of the microsoft's logos plus a written disclaimer. Unless you start selling microsoft things (or other product in the pretense of windows) then it might going to be a completely different matters, and besides *in my opinion* unless your site is making alot from it or had become very noticable, pursuing legals against such is a waste resource for microsoft, the worst that will come first in the worst of cases is probably a cease and desist email notice
I dont think that it would be a problem, but if I were you I would specially place a line on the site which will read that "This site is my own property and has no relation with the Windows of Bill Gates" I dont think you should have any problem then.
I also dont think you cant get in any trouble unless you should keep all clear in Disclaimer that you are no way associated with Microsoft