Thanks Sammie
@sddfg, djdannyp:
You can also use a tool called "Messenger Aero Switcher" programmed by me. It will turn Aero on or off on the fly, you don't even need to restart Messenger.
In order to use it, you should be on Windows Vista with enabled Aero, and you should have the latest Windows Live Messenger 2009 RC (14.0.8050.1202) installed.
Messenger Aero Switcher: Turn Aero on or off just for Messenger
Free download:
Messenger Aero Switcher for Windows Live Messenger 2009 RC 14.0.8050.1202 (165 kB)
Please report any problems or unexpected behavior you might notice.