Last retail windows with multiple installs

I built a PC for a family member and i'm about to build a work computer, i'll need a install for each and i'm not dropping 300+ on 3 different copies of win 8.

Windows 8 is per computer and i'm trying to find out which previous versions allow you to install on multiple computers.

Can you install an existing Windows Vista or 7 using the same key on each PC without activating them and then do the $40 Windows 8 upgrade on each? That's about the price of a single Windows OS.

TeknoBug said:

Can you install an existing Windows Vista or 7 using the same key on each PC without activating them and then do the $40 Windows 8 upgrade on each? That's about the price of a single Windows OS.

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That's a good idea, but sadly i need a disk, my computer came with windows from dell.

I think you can download a legit ISO from Microsoft (or Digitalriver) and burn that or place it on a USB stick and still use the key.