The logic in dual booting operating systems in Microsoft is easy as long as you install the older operating system first. Just like what I recently did with my Lenovo S10 where I triple-booted it with Windows XP, Windows Vista, and Windows Server 2003. I first installed the oldest operating system – Windows XP, followed by Windows Server 2003 and Windows Vista; but of course, each operating system needs to reside in it’s own partition.
If you have downloaded your Windows 7 public beta already, go ahead and you can dual-boot it with your current Windows Vista.
Below is a step-by-step guide written by Lifehacker (link here) on dual-booting Windows Vista and Windows 7.
Article on hard drive partitioning:
Video on Hard Drive Partitioning for Windows 7 Dual Boot with Windows Vista










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In this article you will find a detailed tutorial about how to dual boot Windows 7 and XP without burning a DVD for Windows 7 installation. Practically you can install it directly from XP and all applications installed in XP works in Windows 7 without reinstalling them: just create shortcuts.You will be able to dual boot both XP and Windows 7 , without loosing any data.
http://www.downloadtube.com/blog/2009/01/13/dual-boot-windows-7-and-xp-easily-windows-7-installation-directly-from-xp-without-a-livedvd/