I looked on Amazon and the complaints are when people bought the upgrade it error’d out saying it was no good. What they were selling was CD’s for people who load pc’s to sell so they had the license. MS doesn’t sell the original disks anymore. If I borrowed one (which I tried) it said it wasn’t good either since somehow the hard drive is registered(?) - so what if someone bought a new hard drive? They’d be out of luck. MS is nasty.
Any computer repair shop can get you Win7. Home Prem runs about 90-125 US,,, As I recall, you are an Ozzie gal, but no doubt there is the equivalent of a back alley shop or redneck engineer thereabouts who should know how, and can order you up an OEM disk which will give you the key too... Ozzies are no different than Americans when it comes to rugged individualism, so I find it hard to believe there is no one there who knows how... Tho it might be the vegemite effecting their sensibilities :)
I don't know about 'registered HDD' - I do this kind of thing all the time, and never ran into it, unless you are talking about the hard drive being encrypted, which can still be done... Just would have to zero the drive out completely.
Lastly, look into MS 'Anytime Upgrade' - I don't know if you must upgrade to Win8 or if you can upgrade to a better Win7, but that is all done electronically.
I really don't want to suggest it tho, because I generally hate 'upgrades'.