You can download & install it free from MS, re "XP Mode is a free add-on for Windows 7 Professional, Ultimate, and Enterprise — Windows 7 Home Premium and Starter don’t qualify.
Using Windows XP Mode, you can run programs that were designed for Windows XP on computers running Windows 7 Professional, Enterprise, or Ultimate editions.
Windows XP Mode works in two ways—both as a virtual operating system and as a way to open programs within Windows 7. It runs in a separate window on the Windows 7 desktop, much like a program, except it's a fully-functional, fully-licensed version of Windows XP. In Windows XP Mode, you can access your physical computer's CD/DVD drive, install programs, save files, and perform other tasks as if you were using a computer running Windows XP."