I think I know what happens, Divx is not completely compartible with Aero as Limewire, so when it Runs Aero is disabled. You should select not to run it on startup or update it to a newer version.
well vista should switch away from aero when you open Divx and it should switch back when you close it. So, make sure that Divx is really closed by looking in the task manager and ending any of the divx related processes.
Hmm, I downloaded DivX from their website and have not had an issue. Vista used to warn me via the UAC when it started, but adjusting the security settings for it fixed that.