Tweaktown published a simple guide on how you can install Windows 7 beta and still be able to boot Vista and if you need XP.
Quote from the guide: "In this quick and simple guide today, we are going to show you how to multi-boot Windows 7, Windows Vista AND Windows XP. Actually, the method we use will probably work for booting other operating systems as well, such as Linux Ubuntu and others. This guide will assume that you have Windows Vista SP1 (yes - the service pack must be installed, too!) already installed and you are prepared to upgrade that install to Windows 7 (W7) BETA, as per the requirement from Microsoft. It's going to mean installing Vista again once W7 has been upgraded, but this is the safest, quickest and easiest method we have found. We do know there are many software applications out there that allow you to setup dual booting, but this method doesn't require any additional software at all and just works!"
Read the full guide on Tweaktown.