Hi mistu4u - Welcome to w7forums
I'm not sure adding sub-forums for older OS's will help much. There are many forums that specialize with the older OS's. This forum was created for helping with Windows 7.
Personally I don't have 98, 2000, XP, 2003, or Vista. I'm afraid my memory is not what it used to be and having the OS would be a big help when it comes to helping others. I do have Windows 7 and will help users of Windows 7 where and when I can. I feel adding a sub-forum for older OS's would be a bit redundant when there are so many websites that are already well established.
However in the end its not my decision to make, the decision is ultimately up to admin to decide what is best for the forum. Whatever the outcome thanks for the suggestion.