10.4v is pushing it... you probably can't crank after 10.4v, which is why it's dying.
You should have power to your computer completely cut when you're not using it. If you want to save your state, use hibernation. Do not run an entire computer (even in standby) on your car battery for more than... say... half an hour?
Most PSUs have appropriate settings for your car. If your car gets parked for 13-15 hours fairly often, youll probably want the power to cut after, say, 10 minutes. Your startup time is only a few seconds in most cases...
And lastly, why the hell are you running vista on a carputer?