What you have is an upgrade only copy of Windows 7. It allows you to do a semi-clean upgrade from XP or Vista to Windows 7. It DOES NOT allow a completely clean install - you must install over the old XP or Vista installation. Then it will activate. On a clean disk it will NOT activate.That said - if you call Microsoft and explain that you have build a completely new machine and why - Microsoft can (but does not have too) activate Windows 7 for you.I went thru the same process as you and since Microsoft had a record of me they did activate Windows 7 for me.