Driver conflict with the OS?
Have you booted in safe mode to see if it loads fine with that? Test that a few times and see if you still can't load windows.
Not sure when you started having these issues, but if you have a restore point prior to these issues occuring it wouldn't hurt to restore the computer to that point. You could also update all of your drivers to ensure you have the latest versions. Also update Vista to the latest updates. Since Vista does not come with a repair console, if a restore doesn't work and updating the drivers does work my suggestion would be to reinstall the OS.
Since you are having this problem on when the OS is loading, i believe there is a driver conflict.