You could either find a qualifying product perhaps second hand that will satisfy the upgrade requirements or ring Microsoft and ask them what you can do. If it hasn't been activated they may be inclined to be helpful but its hard to know.
1. Install win 7 normally, but do not enter the activation code - skip that step.
2. Install winows 7 a second time. make sure to have it overwrite the first instance. This time, enter the activation code.
I've done the same thing... I have a valid OS system license from XP and the windows 7 upgrade. But idioticly only used the upgrade on my fresh harddrive. Which activation code do i use on the second install of windows 7 upgrade? the activation code from the XP OS or the one for the windows 7 upgrade?