Thank you all for your replies, and the sanity check. I didn't get back to this until just now, but I repeated my steps to download the Windows binary and this time everything went off without a hitch. Same browser and everything. Impossible to know whether it was a glitch on their end which they quietly fixed, or some random "internet thing" no one on either end had control over. I spent too long in the software business to get very worked up about a minor problem like this.
Thanks again, fellow kids, and how do you do.