Today Microsoft released the latest version of its virtualization software, Virtual PC 2007, now optimized for Vista.
Virtualizing software like Virtual PC and Microsoft's rival in this arena, VMWare, allow users to run older operating systems, or alternate operating systems from within Windows.
Volume license customers running Windows Vista Enterprise Edition are allowed to run up to four additional virtual instances of Windows on their PC. Other users can download Virtual PC, but need to license any other copies of Windows, however, MS offers a version of XP with IE 6 for free download that will expire in March. This is meant to allow web designers to upgrade their current XP to IE7 while still maintaining a way to test for the old browser.
VMWare, my personal preference, also provides a tool to convert an existing windows installation into a virtual machine, so if you want to keep you old computer because it's got everything installed already, but it's just taking up too much room in your home office you could try virtualizing it.