Solved, i put the solution here if it can do any good.
Changed GPO: Computer Configuration\Administrative Templates\System\Specify setting for optimal component installation and component repair
To: Enabled
with the option: Contact Windows Update directly to download repair content instead of Windows Server Update Services (WSUS)
Maybe this is due to that i did set up this server as a replication ad in a domain controled by an previos axisting sbs2008 server, telling everyone in the domain to use the sbs2008 wsus service. I think i had to set up the new win 2012 server this way to avoid having the sbs2008 server shutting down due to the apperance of another domain controller.
The changed GPO made it possible to install .NET 3.5