The Avalon team has dropped a new Customer Technology Preview (CTP) release that is now enabled to run on XP and above.  This version runs on Whidbey Beta 2 (build 2.0.40607.51), which is older than the most recent Whidbey CTP (released in October).  Those two versions of Whidbey probably won't mix well, so I'd recommend installing on your Beta 2 machine. 

The Avalon team has been working really hard to get this build going, congrats to them!