Lifehacker's Adam Pash wrote a guide for testing the Windows 7 beta on a machine already running XP or Vista. It walks you though adjusting the partition size of your current OS so you can test the new system out without dumping your current setup.
Before you can get started, though, you'll have to download the Windows 7 ISO image an burn it to a DVD. Unfortunately, the download links on the Microsoft site have been temporarily removed, apparently due to the volume of download demand. Links to the official 32bit and 64bit downloads were available for a brief time, though, and they were posted to a number of blogs. I've posted what appear to work below.
You'll also need to get an activation key or the OS will expire in 30 days. This is also unavailable at the official download site, though the Neowin blog has recently posted an alternate way to obtain the activation key as well.
So, to sum things up:
- Download the Windows 7 Beta ISO (warning: big 2 gig file): 32-bit Version or 64-bit Version
- Get the activation key using Neowin's tip
- Set up dual-boot with Adam's instructions
Note that the activation keys are said to be limited to 2.5 million copies. It might be smart to grab one now, even if you weren't planning on downloading or installing the OS right away.
Windows 7 Beta - Official site. The official download process should be available again soon.