UPDATE 27/03/2013: If you simply want to upgrade your solution to SP2013, just dubbel click on the sln file on a dev server where SP2013 is installed.
UPDATE 21/03/2013: If you run this command you are able to start SharePoint 2010 solutions in Visual Studio 2012, as a side effect we are no longer allowed to create SharePoint 2013 solutions based on the templates.
Removing the added keys allows you to create SharePoint 2013 solutions, but than disables the SharePoint 2010 solution templates. Seems like we cannot have both…
This is due the fact that Visual Studio 2012 checks the “HKLM:\Software\Microsoft\Shared Tools\web server extensions\14.0\” registry if it contains the string value named “Location” before it allows you to continue.
If you run the following command inside your PowerShell window, the value will be set correctly.
Set-ItemProperty -Path "HKLM:\Software\Microsoft\Shared Tools\web server extensions\14.0\" -Name "Location" -Value "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\"
This location exists on my SharePoint 2013 development environment.
If you changed the location of the rootfolder during installation, please use the correct location here.
After you closed Visual Studio and restarted the application you should be able to continue.
Hope it helps!