I'm not as familiar with Vista as I am with XP, but you can modify the boot sequence from XP. Go to the Control Panel -> System -> Advanced (tab) and choose "Settings" from the "Startup and Recovery" section near the bottom. Near the top of the window that pops up, you should be able to set the default OS to boot into as well as the timeout. This way you don't have to install any other programs.
Like I mentioned, I haven't tested this with Vista, it may not work.