Performing a clean install from an upgrade disc is possible even without the need for a setup disk from a qualifying older version of Windows. With Windows 7 ("upgrade" copies), there are several ways of going about this. The one that I used involves installing a trial version (which does not require entering a product key), and then performing an upgrade installation on top of that trial version. This method is time-consuming, but reliable.
And Microsoft made it a bit easier for such an installation because it provides two additional options besides the one I used: One involves a registry tweak; the other involves simply activating Windows on the first installation after entering a valid product key (typically done with installs from non-upgrade disks). The latter may or may not work, depending on the hardware configuration although the likelihood of such an activation succeeding is very good if you are reusing the exact same hardware (primarily the motherboard and network card) that you used when you ran a qualifying previous version of Windows (e.g. Vista Home Basic or Home Premium for a Windows 7 Home Premium "upgrade") that had been activated.