I converted my VS2005 to VS2008 and I get the following error:
error MSB4075: The project file must be opened in the Visual Studio IDE and converted to the latest version before it can be built by MSBuild.
Any help appreciated. Google failed me this time lol..
It sounds like you converted the solution, but not the project file. Maybe the project file was unloaded when you did the conversion? At any rate, try opening the project file in Visual Studio and let it convert it.
Rob Prouse is right - it sounds like the project file hasn't been converted. If you can't follow his advice to convert the project file for some reason, you should be able to run the old version of MSBuild manually. MSBuild is installed with the .NET framework, and should be found at %WinDir%\Microsoft.NET\Framework\v2.0.50727\MSBuild.exe. From memory, the command line is just "MSBuild ProjectFilename".
Like Odez said in his comment, I also got this error because of .user files. In my case it was an older converted project which got the .user files checked in when converting to svn. Removing them from version control fixed the error.