Okay, it's definitely lsxcommand. I've screwed up my registry to the point where all drives and directories open in xplorer^2, even if I'm in an explorer window when I open the folder/drive. It works in the Windows Run Box, it doesn't work in the lsxcommand on my theme.

Well, at least I have a more refined question, now: Does anybody know how lsxcommand decides to handle its input, how it decides what programs open what, and where that can be changed? I'll go through the documentation again and post again if I find anything.