If you want lots of users, be prepared to buy a server-class machine.
Concurrent users means how many people are on the server at the same time. You don't need a big box if you have 1 million users but only one online at the same time. You do need a big box if you have 1,000 users and every single one of them is online all the time. See the difference?
In my opinion, don't go with windows. Use LAMP, which is Linux - Apache - MySQL - PHP. They all come together as a package that way.
Are you prepared to manage your server? Because to be frank, it doesn't sound like you are. You might be better off leasing a dedicated server and paying someone else to manage the software, and you focus on running your board and marketing and so on. By doing that you can start with a smaller hosting package and see if your board really takes off before investing the big bucks in a major server.
For example, my biggest board is currently hosted on a dual CPU Xeon box. But we didn't start out there. We started on a single Celecron CPU. That was plenty for our first year of activity.
Apache and MySQL and PHP are all available as a free download. All are available to run under Windows. But unless you want to spend your time learning how to install / manage / upgrade these packages, I think you should at least investigate a managed hosting solution. Thus the ever-popular suggestion of going to www.webhostingtalk.com