Just for your information: I downloaded and tried the public beta 1 of Windows 7 on a test system: Intel Pentium D 2.8 GHz, 4 GB RAM, dual monitor configuration. The OS works without problems but MA3 beta 8b doesn't work: after initial logo, both monitors come black and I have to press Esc to exit.
Are you sure that you have DirectX9 installed? I seem to recall reading somewhere that Vista/Windows 7 only install DirectX10 by default, and 10 is NOT backward compatible with 9. (e.g: Having X10 does NOT mean you can run X9 programs; you actually have to have both X9 AND X10 installed!)
~Ralph S.