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
For additional assistance with the migration of Windows XP to Windows 7, please go here: http://tinyurl.com/mhbep4
Microsoft Windows Client Team