I've recently had to uninstall/re-install my .NET Framework 4 after finding my current installation was corrupt. I found this when attempting to upgrade my Telerik Controls.
I manually uninstalled my .NET Framework using the command line and then re-installed the newest version of the framework from MSDN.
Now when I open previous projects in VS2010 (which are coded in .NET 4), it prompts me to Install .NET Framework 4.0 or retarget the Project to a another Framework or Profile.
Is there a way to refresh Visual Studio 2010 to recognize that I have the .NET Framework installed? and/or would a simple Repair on my Visual Studio installation correct this problem?
The first thing you should try is to repair your Visual Studio installation. VS2010 relies on .NET 4.0 as a prerequisite. When you re-installed .NET 4.0, you probably messed up the VS2010 installation.
Most likely fixing VS will work, but make sure you apply all SPs and recent patches after you do this. In the future, you should try to reinstall .NET instead of uninstall/install, as you can do a repair install.
来源：https://stackoverflow.com/questions/6389368/visual-studio-2010-not-recognizing-net-framework-4-0