[MovedGoalPosts]

You can always remove the windows version you install and replace it with something else. Very simply put you'd just do the replacement OS install having formatted the HD as part of that install.

As for M$ not giving you a code, call their bluff and let them tell you that you are not allowed to install your old OEM disk onto your new PC. I can't see how they can tell that you haven't resued some of your old components in building your new PC. You can always claim that you have reused components, say your graphics card, network card and the like which were purchased as part of your earlier computer. But you wont know until you try, and as said above, you may not even need to call M$, depending on the number of times the disk was originally installed and activated.