What is this in VS2010?

 Could not resolve mscorlib for target framework '.NETFramework,Version=v4.0'.  This can happen if the target framework is not installed or if the framework moniker is incorrectly formatted.

I just get latest version from onshore client using SOS. Project is in VS2010. I compiled the project and all of a sudden I get following issue.
Solution compilation failed but with no error.
If I delete WCF reference following error appeard.



I unlinked the project from source repository, then manually deleted the reference from folder (from VS it still not allow me to delete the reference). then Reoped the project, added fresh WCF reference, bilt the solution afterwards. It worked untill I checked in the code again to VSS using SOS.

Then I repaired the  existing VS 2010 installation. But still not succeeded to resolve the issue. Every time I have to perform above practice.

Now I am considering to uninstall the existing setup, and install fresh VS2010 copy, Will check if problem still occur. (Every thing is working fine on another developer's computer).