And are AAC files 'standard' and playable by most third party media players?
Yep, as far as I'm aware AAC will be catered for by the majority of players. I obviously can't guarantee obscure players will support it, but from what I've seen the few remaining non-iPod players that still sell well either run Android or are Sansa clip products, which all natively support it.
I built a PC a while back and am using Win 8 with an OEM license (which should permit transferal between computers).
I'd like to switch my 500GB HDD for an SSD (specifically the Samsung EVO), would it be as easy as switching the drives and installing windows from disc? The authentication code should work again?