Is there a secret trick to force antialiasing inside Viewport3D in Windows XP?
Have you tried this (from your thread on MSDN forums)?
Well, it seems the reference in the MSDN link above incorrectly specify the affected registry root key. In MSDN it is specified as HKEY_CURRENT_USER, while the correct root key should be HKEY_LOCAL_MACHINE. I've tried setting up the HKEY_LOCAL_MACHINE\Software\Microsoft\Avalon.Graphics\MaxMultiplesampleType to '4' and I can get antialiasing for my WPF Application on XP.
The feeling I get from Matthew MacDonald's Pro WPF Windows Presentation Foundation in .NET 3.0 is that it's not possible:
There's one exception to WPF's software support. Due to poor driver support, WPF only performs antialiasing for 3-D drawings if you're running your application on Windows Vista (and you have a native Windows Vista driver for your video card).
I've never seen anything to suggest that you can enable AA in WPF 3D on anything but Vista, but if there is a way it's new to me and I'd love to know as well!