I don’t have vista, but frame rates are extremely finicky. I am constantly fighting frame rates, with a quad core and 3 mb of ram. Who knows. The blender GE is not as robust as other engines, in part because it’s a non-compiled engine. The havoc/unreal etc engines need to be compiled, and compiled programs always run faster. My knowledge on this subject is limited, but if you’ve ever played a 3d game on your computer, and it runs fine, and you get a terrible frame rate with just a plane, then you know it’s not your computer. It’s the engine.
Your graphics card may also be the issue. Apparently Blender doesn’t like ATI. I have an nvidia 7100 something, and I can play bioshock, oblivion etc fine, and it struggles with a simple scene.
Check for lots of lights. Lights are always a framerate killer. Also make sure there is not extranious geometry. Of course, as I understand it, vista is a hog. I got new computer with xp on it recently. No vista for me.