I've tripped over this issue a couple of times so I figure I'd offload it to my blog to free up some space in my head.
The short story:
If you've just installed a new web application to your IIS server and it uses ASP.NET and you're unable to view the web page (specifically you may instead be asked to save a file) then it may be because you need to (re)register ASP.NET with IIS.
You do so like this:
> cd "\WindowsFolder\Microsoft.NET\Framework\VersionNumber"
> aspnet_regiis.exe" -i
The longer story goes something like this:
You install the .NET Framework version x.y. However you didn't have IIS installed in Windows. Then you install a web application to your IIS server . Viewing the web app pages  confusingly just doesn't work, dammit! Then you remember the magical register command and voila, birds start singing, sun beams out from behind the clouds and the world is a happy place once more. :)
MS have also documented the issue.
 For me it always seems to be CruiseControl.NET.
 Which, for CC.NET, is http://localhost/ccnet/default.aspx.