Windows only: If you're curious whether your processor will support XP Mode in Windows 7 or not, SecurAble is a simple freeware application you can use to find out.
SecurAble is a tiny app from Steve Gibson of the SpinRite fame. When you run the standalone app, it returns information about three features on modern processors: Maximum Bit Length (32-bit or 64-bit), Hardware D.E.P (provides protection against malicious code), and Hardware Virtualization (optimised processor instructions for virtual machines).
If you get a big green Yes in the third slot, your processor supports Hardware Virtualization. If you meet the other requirements—1Ghz processor, 1.25GB of RAM, and 15GB of disk space per Windows virtualization—it's all system go. SecurAble is freeware, Windows only.