if you are already using it, did it come with a computer already installed or do have the disks still? if you have the cds then you can install it up to 2 or 3 machines - from memory.
otherwise I searched on pcpartpicker for Australia and this store was the only one that popped up with win 8.1 in stock
http://www.skycomp.com.au/microsoftw...ingsystem.html (note: I am unfamiliar with this store).
same thing again, once installed turn off automatic updates. win 8.1 actually does what you tell it to do unlike 10.