As I wrote in here:
http://forum.avast.com/index.php?topic=21312.msg180265#msg180265Oh yeah... almost forgot... avast! works as a charm with Vista, but the only problem I could find was you are unable to update program and virus definitions by accessing those options from system tray. You get a note from avast! that these functions are not supported yet. But you can update everything through GUI without any problems.
Manual update through the system tray options is not possible at the moment. Remember, Vista is still under heavy development, so do not expect that everything will work perfect. Once Vista goes gold, all these things will be addressed and fixed.
For now, you can still manually update avast! (not through system tray icon), at least I can do that without any troubles. Just open your avast! program, and invoke manual update through the GUI (skin)... option is called iAvs. When you do that, avast! doesn't report any messages/errors.