Note: Microsoft's states that Windows XP users will need to do a clean install. Also going from any 32bit version of Windows to a 64-bit version of Windows 7 will also require a clean install. "Microsoft designed Windows 7 Upgrade media for Windows Vista. A customer with Windows XP can purchase Windows 7 Upgrade media but must back up their files, clean install, and then reinstall their applications."
However I have found a path from Windows XP to Windows 7: (that may work for some XP users): "In-Place Upgrade Windows XP to Windows 7" http://www.pagestart.com/upgradexptowindows7.html (This article will be updated as more information becomes available)