HOW TO: Dual-boot Windows XP and Windows Vista
First you have to download the Beta 2 of Vista available at Microsoft.com, or order the DVD kit to 
be shipped to you for about 10 bucks. Last time I checked everyone and their sister was downloading the 3.12 GB file from the Microsoft servers, so for awhile there they took down the download link and you could only order the DVD kit. I got in on the download early which took me several hours to complete. Don’t do the download with your browser, folks - use a download manager or wget. It’s a monster file and you will hate yourself if it fails halfway in and you have to start from the beginning. UPDATE: If the download is unavailable on the Microsoft site, give the torrent a try after you get your Product key. It’s an .iso file, which you have to burn to DVD. (So, obviously, you need a DVD burner.)
Next I started up the install and found that Vista will only install itself on a primary partition. If you choose the C: drive where XP is already installed? Vista asks if it can rename your Windows folder "Windows.old." Uh, no thank you Vista. To get dual-booting to work, you have to create another primary partition where Vista can live alongside XP in peace and happiness. Windows Vista Beta: How to dual-boot Windows XP and Windows Vista - Lifehacker