TechSpot posted Dual Boot Windows 7 with XP/Vista in three easy steps:

Windows 7 has proved to be quite the drastic improvement over Vista, enough that it even has XP stragglers crawling out of the woodwork to check it out. Your chance to test drive the release candidate is now running thin, in fact, if you haven't tried the new OS until now you are better off postponing your plans for a week and installing the real thing. The final version of Windows 7 has already been sent to partners and is scheduled to debut publicly later this week on October 22.

Even if have pre-ordered Microsoft's latest OS, installing it on top of your existing copy of Windows may feel a bit premature if you haven't been running the beta or RC as your main installation. Thankfully, setting up a dual boot configuration is both easy and practical. If you're looking for a quick and dirty, yet thorough how-to on getting Windows 7 to run alongside your installation of XP or Vista, read on.