Dave,
This is the information listed on the product detail page:
"This is an upgrade product. To install this operating system, you must have a valid license that is not in use on another computer for one of the following operating systems: Windows 2000; Windows XP Media Center, Tablet PC, Professional, or Professional x64. Upgrading from some of these systems requires a "clean install," meaning you must remove all files and applications, install the system, and then copy back the files and reinstall the applications. For the rest of the systems, you can install "in place" as long as you have the latest service pack for the current system. For complete information, see Microsoft's Upgrade Paths page."
Autumn