I know you seem to have made your decision on this, but I thought I'd throw in some input too.
Although I may not be as active of a poster as I once was, I still take time daily to read through threads and such. I have noticed a fair amount of threads regarding PC's, Windows issues, etc. including, but not limited to running Windows on a mac. I think it would certainly be a useful endeavor to create a windows board so that users have an obvious location to post their questions. Many posts regarding the above end up in "Anything Goes," adding to the cluttered atmosphere of that board. Also, many of those posts start with "I really wasn't sure where to put this."
You certainly have a point in mentioning that there are numerous PC forums that can handle issues specifically relating to Windows. However, I don't believe this is a valid excuse not to include Mac-Forums in this. There are numerous forums in which Macs are discussed, several of which have more members that Mac-Forums. It is time to realize that the distinction between Mac and PC is closing rapidly. Apple, although not officially supporting Windows, is certainly giving some credence to it, hence Boot Camp. Numerous mac users also own PCs, and they should not have to go elsewhere to get half of their questions answered. The polite and intelligent atmosphere of Mac-Forums is well suited for organized discussion of both operating systems.
Mac-Forums has a large volume of posts relating to windows, involving both the use of windows on Macs, and the use of windows on standard PCs. These posts end up scattered around the forum because there isn't a truly proper place for them (and Windows certainly qualifies for more than "Other Software."), and a new board would be well suited to handle them. If, upon creation of a new board it is discovered that you are correct about the lack of volume regarding windows, then it is a simple matter to dump the threads into the "Other Software" board, and to close the new one.