Thanks! The developer runtime you linked to didn’t work, but it got me to the right place. There is now a Silverlight 5 release candidate download. Works perfectly!
Steve,
Many thanks. Worked perfectly. The problem for me was that I just assumed that setting the compatibility setting to XP SP3 would set an exception for DEP. I never checked to make sure that it had.
Now I know that it doesn’t, and your very thorough post will probably help someone else eventually.