I am running VS2005 Versions 8.0.50727.762 (SP.050727-7600). When trying to open a project created by another developer (Im not sure what his version of VS2005 was), I got the following error:

The project file "C:\path\to\project\file.csproj" cannot be opened. The project type is not supported by this installation.

I tried a few things from this discussion, but with no success http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=931459&SiteID=1
Finally, I opened the csproj file in notepad and compared it to another csproj that opened correctly. The broken file had the following node; I simply removed this node and was able to open the project successfully.

<ProjectTypeGuids>{9E5D3E2D-E4E2-418e-8D80-2F0DA9A94F9A};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>

Note: I have just found a related discussion: http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=629702&SiteID=1