Use upgrade CD for a new OS?

One of my buddies at work was telling me there was a way that I could use one of the upgrades from my Windows 7 upgrade pack to download as the new operating system in my new arcade build.  Is this possible? I really don't want to spend $120 on a windows OS when I can just use an upgrade from the CD pack. 

registry hacks make is possible, I think... i've heard of it anyways...

IDK abotu the anytime upgrade within win7, but the xp/vista to 7 is possible woth reg hacks


Jtgreen you can get windows 7 for $80 or so dollars on amazon. If you wanted to.