Okay, I've found the solution.
Use FinalBIG to extract the *.vp6 files from the *.BIG archives. However these VP6 files won't play in On2's TrueCast player, nor the FLVPlayer program, and their desktop codec pack doesn't work with these files either, but you need it to play the *.avi files later on.
I then followed the rest of this guide to convert them from *.vp6 to AVI files which play fine.
http://www.ppmsite.com/forum/viewtopic.php?t=14303