RSLinx may be the issue, but I would also try compatibility mode if you haven't already.
Try running the C:\Program Files (x86)\Rockwell Software\Studio 5000\Logix Designer\ENU\v21\Bin\LogixDesigner.exe in compatibility mode with Windows Vista Service Pack 2.
You could also try what I do, set C:\Program Files (x86)\Rockwell Software\Studio 5000\Launcher\Launcher.exe to run in compatibility mode with Windows Vista Service pack 2. Since doing that I haven't had any issues with any of the versions.