You simply can not beat vBulletin. With that said, it isn't free, so not always an option.
Of the free forum suites out there, my favorite is
(IPB) Invisionboards. It has the most (and most needed) functionality out of the big names most people will probably suggest. The closest runner up currently (imo), and also just as popular, is phpBB (already mentioned). Until phpBB releases phpBB 2.2 though, it will lack many popular features found in other forum suites.
Both of the above use PHP and MySQL (possibly other DB's available), but assuming those are both free and run on windows, I would assume they will work for your windows hosting. If not, you will need to go with an ASP solutions most likely, which I have never really dealt with. I have seen many of them though and they don't seem to be quite as modern and feature rich as the PHP/MySQL brand, likely because of the community backing one obviously has over the other
For a pretty recent and decent article on this topic check
this out. It's a forum software review on sitepoint.com. Even if it is a little overly opinionated, you still get a look at most of the features
It includes PHP, ASP and CGI (in this case perl) solutions.