Hmm, offhand I'm not sure. It may allow you to install both. If it doesn't, then I would install the older one, copy the assemblies needed to another new References20 folder, then uninstall. Next install the newer version. Hopefully you'd still be able to point your old projects to the References20 folder and pull that one in without the 3.0 version being pulled from the GAC. If 3.0 is being pulled from the GAC inappropriately, then make a References30 folder and copy the Shared, WinUICore, and Wizard assemblies for 3.0 to that and uninstall v3.0. Change your v3.0 based projects to use the assemblies in that folder.