I happened to reinstall Visual Studio 2010, then when I opened an existing solution that contains a Class dll project and a web project, I got an error "The project type is not supported by this installation" for the web project only. So when I looked in the .csproj file in edit mode in Visual Studio, found a red line under <SilverlightApplicationList> </SilverlightApplicationList>. I removed it and tried again. But then <SccAuxPath>SAK</SccAuxPath> got a red line. Since this property is about TFS, spent some time by googling TFS project setttings (since I also reinstalled TFS), but in vain.
Finally it tried all other projects and noticed only web project had this kind of problem. It turned out that I did not install Visual Web Developer component when installing Visual Studio 2010. There are many reasons of this error "The project type is not... installation" but error messages in VS project edit mode was kind of confusing.
After adding Visual Web Developer in Add/Remove Program, it worked fine.
