If you are starting to develop a new web application from scratch and the only two requirements you have is that it will be deployed to Windows 2008 Server and that standard ASP.NET incl. VS2008 doesn't cut it (doesn't matter why, let's say it's the license cost, you won't be hosting Windows 2008 Server yourself). Which web framework would you choose under this circumstances?
I'm stuck wondering why you'd host on a Windows server and not use ASP.Net. It is the best of the server-side frameworks for a windows environment, as it works most intuitively with IIS.
But there's another problem there: If memory serves, all the other frameworks require additional software to run on the server. PHP requires PHP runtime to be installed as a CGI Extension in IIS, Rails requires Ruby, CGI style options (C, Perl, Python) require not only the compiler/runtimes but also some poking under the hood in regards to how IIS handles those files, JSP requires... I'm not sure, I've never used JSP, and ColdFusion requires that framework and it really is a hefty licensing cost (as opposed to ASP.Net which is a free download for the server).
So, in the end, you're left with basically picking an option, installing the necessary software and going forward. Personally, if forced to make this choice, I would choose either PHP or Ruby on Rails, but there's no valid reason for that outside of a random, and subjective choice.
I read a benchmark article some time ago that suggested running an open source web stack on top of Windows was the best performer out of all the variations (i.e. WAMP). I don't know how much faith to put in those types of articles, and IIRC it beat out LAMP by some really small margin. You might find WAMP to be a good fit for your situation, but I think you should save yourself the Windows tax and use a LAMP (or use Postgres instead of MySQL... I like it better).
I agree with Stephen, seems a bit strange to blame licensing costs for not using ASP.NET especially when you're going to be using Windows to host? Admittedly VS 2008 Pro isn't cheap, but it hardly breaks the bank either. The new 2008 Express editions of VS are free and I have to say aren't half bad.