One of the key features of Windows 7 is its ability to run an 'XP Mode' for older games like Starcraft that can't cope with a 64-bit environment. It means booting into a Win XP desktop without leaving Win 7 host, and there are just two problems: to install it you need to a copy of the very expensive Ultimate edition of Win 7, and it can't use the 3D acceleration of your graphics card.
Since most of us have a copy of XP kicking around, there is a way around this. There are two other major virtualisation applications, VMWare www.vmware.com and VirtualBox www.virtualbox.org. Both are free and, once installed, come with simple instructions for installing XP over the top of Win 7 without needing to dual boot.
What's more, they both support 3D acceleration of virtual machines, although it's not a completely reliable feature yet.
My personal preference is VirtualBox: it's much simpler to manage thanks to a straightforward control panel interface. VMWare uses a clever web-based system which is great if you're a sysadmin but overkill for single desktops and harder to learn all the technical details of. I've also found VirtualBox generally a bit quicker and more reliable.