Okay, simply running deinstall on the Divx items on the PC seemed to work, didn't even need a reboot. Both the Windows control panel and GSpot no longer report any Divx codecs.

Try MPEG Streamclip on the PC again, same exact crash as detailed at the top of the thread.

Of course the whole "it crashes in the Divx codec" thing was only diagnosed on the Mac, I was just hoping that what was good for the goose was good for the gander. I'm going to try removing Divx from the mac (not sure how but I'll poke at it) and see if I get the same result.