Let me expand on my answer.
If you bought the retail version of Win8.1 you probably got both the 32bit and 64bit install disks.
Since you dont have the disk I'm assuming you didn't buy the retail version.
If your other pc is an OEM, like Dell, Hp, Lenovo, then that version of windows is tied to the motherbd and can't be moved to a new system; you need to buy a new copy of windows.
perhaps if you explained your situation more clearly we could better assist you.