i disabled framebuffer effects and steam overlay and i was finally able to play 2h straight without any crashes.
I know it's a comprimise that shouldn't have to be made, but it sounds like you might be able to enjoy the game if you turn
both those things off again. Thanks for being patient enough to test all those drivers (I know the whole reboot!reboot!reboot!reboot! thing isn't fun), and thanks for deciding to keep the game. It makes me want to continue trying to help. I'll be interested to hear how it works for your brother.
Another long shot: Since you said the demo "didn't crash as often", I assume it did crash. So
something's making it crash, and it's not necessarily Steam. There may be some program somewhere on your computer that is trying to pop up overlays that interact badly with Aquaria's framebuffer effects. This might be anything from Steam to your computer's volume control to certain virus scanners. (Maybe Aero, but I don't know anything about that.) If the full version still crashes with the two things in the above quote disabled, and if you have any patience left, you might try finding overlays in other programs to disable. (Just don't disable a virus scanner completely. Find the overlay option if it has one.)
Sorry if I'm sending you on a wild goose chase here, but I'd be going through all this myself if I had the problem, knowing how good Aquaria is.

Or if you're tired of testing things, here's to hoping it works in the future!