If you have Enterprise, Ultimate or Pro Windows 7 you can download XP Mode free from Microsoft: http://windows.microsoft.com/en-US/windows7/products/features/windows-xp-mode/
If you have Home Premium, you can use VMWare Player to accomplish the same thing as this site has step by step directions: http://www.howtogeek.com/howto/11060/create-an-xp-mode-for-windows7-home-versions-and-vista/