Was I supposed to mention I'd done that? Mylenium could've asked if I had older programs installed or whatever, rather than stating it as if it was fact (and in doing so, making himself look like an arrogant *******; like I wrote, he's been providing unhelpful arrogant advice on every forum he's answered one of my questions, which contributes to my bitterness towards him).
Anyway, I still have no idea what the problem was, but I did get a solution:
I downloaded the installer again through the Adobe Downloader Assistant, but this time after the download was completed and the files were decompressed, it automatically launched the installer (didn't do that the first time I downloaded it). So I installed it, and I didn't run into the "please insert disk" problem.
Still, having to re-download seven GB which I already have is a bit annoying.
