This is now Arena 4.1.2 Keeps Crashing.
I noticed if I changed decks to quickly it would crash. If a deck hadn't finished loading it's previews and I changed decks it would crash every time. Other than that it would just crash changing decks.
Aero was off. After seeing this post I turned on Aero and tried again. This time it works. It hangs for a second, then starts changing. Rather than just hanging.
Win7 64bit, 8GB ram, Geforce 5850.