For the first time I was trying to debug my silverlight application projects on Windows vista. Every time I was trying to load test page of my web application, i kept getting Error 2104, telling me that download of silverlight application is failing. For a moment I thought it may be settings in my browser that are preventing download of this silverlight application. I could not find anything strict in my settings that will prevent the download. Then i stsrtaed digging around and found that silverlight application has new file extension type ".xap" and its a new mime type as well. So I added a new entry in MIME types in IIS and there you go. It worked like a charm. Just add the following entries and you will be all set.
File Extension: .xap
Mime Type: application/x-silverlight-app