Vista is supported but it must be ultimate, business, or enterprise. (
http://www.graphisoft.com/support/archicad/system_requirements/ )Video cards/drivers can be a problem though. On my new laptop with vista business, I had to find and download the latest drivers from nvidia in order for opengl to function and crash AC. The drivers that came with the system or were updated through the system utilties did not work.
On my new desktop system with vista ultimate I am having opengl problems as well but I think that's due to multiple video cards as i posted in the hardware section (and am desperately hoping for a solution, three monitors is great!)