Userunknow,
You can perform a clean install from a Windows 7 upgrade version however you will have to have a valid previous copy of Microsoft Windows XP or Microsoft Windows Vista installed to validate the upgrade and activate the product key. Please note that when migrating from Windows XP to Windows 7 you will not have an "in place upgrade" option. You will however have the option to select "custom" install when prompted. The Windows 7 install process will then copy all of your data in "My Documents" over to a Windows.old folder within Windows 7 itself. All applications and documents stored in other locations will have to be reinstalled / transferred manually.
For more information on the Windows 7 Upgrade, please go here:
http://bit.ly/3DvynK
When migrating from Windows Vista to Windows 7 you will have the option to select "custom" or "upgrade" install when prompted. By selecting the "upgrade" option, your documents and applications will follow and carry over through the install process. If you select, "custom" however you will be able to perform a clean install and all applications will have to be reinstalled manually -- documents will be moved to a folder entitled "windows.old".
For additional assistance with the migration of Windows Vista to Windows 7, Microsoft does have an official Windows 7 Support Forum. It is supported by product specialists as well as engineers and support teams. You are welcome to check the threads there and receive additional assistance and feedback.
Jessica
Microsoft Windows Client Team