The type 'EPiServer.Personalization.GuiSettings' exists in both 'c:\WINDOWS\assembly\GAC_MSIL\EPiServer\5.2.375.236__8fe83dea738b45b7\EPiServer.dll' and 'c:\WINDOWS\assembly\GAC_MSIL\EPiServer\6.0.530.0__8fe83dea738b45b7\EPiServer.dll'
I've got 5 and 6 installed on the same machine, as I am starting a new project. I also have to work on an existing site with 5.2, and now I'm getting this conflict.
I tried uninstalling the CMS6 version as I will not be able to develop on this machine (XP machine, but I need Vista or Win7 for the commerce component), but I get "Error - The term 'New-EPiCMSWizard' is not recognized as a cmdlet, function, operable program, or script file. Verify the term and try again."
Never mind, removed the entries related to version 6 from the registry - HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Installer\Assemblies\Global, then removed the items from GAC manually.
Reinstalling EPiServer Framework should fix one of your problems as described here http://world.episerver.com/Templates/Forum/Pages/Thread.aspx?id=37894&epslanguage=en/