Errors on basic classes after 2012 migration

Hi,

I have opened our asp.net 3.5 website on a Windows 8 64bit 2012 VS Ultimate (formerly 2010 on Windows 7 32bit VS 2010 SP1) and it has migrated the project. Gave me the Upgrade Report with only 2 warnings - no errors.

Now when I open the project I get errors on the most basic classes like DateTime or HTTPCOntext:

"DateTime is not declared" or "HTTPContext was not declared"

I have checked all the references to the DLLs etc but can't see the problem.

I want to run the website on 3.5, not 4 or 4.5 so its not a real migration IMO.

Any ideas?

Thanks!