Windows Vista x64. It defaults to IE 32-bit, and will not allow me to set 64-bit as the default with the "Make Default" button.
I'm currently forced to workaround by starting IE 64-bit manually, and leaving it up with an empty window. Then when links are fired from apps, they open in that 64-bit version.
Problem being is that sometimes I forget and get a bunch of stuff opened in 32-bit without realizing it. Then I have to remember all the links I clicked, close ALL of them, open IE 64-bit, and put them all back in. Trying to avoid all that re-work when I forget to do the workaround.
Unfortunately, all the forums I've googled that discuss this just have a bunch of people saying "don't do that" and "you don't want to do that". Yes, I do want to do this. The particular feature I use only works right in my 64-bit version, and I don't use 32-bit plugins. I fully understand the implications of what I'm doing. :-)