I used speedtest.net on Tuesday afternoon, when I was comparing two modems.
Having read this thread, I ran MSE over the whole computer and it found no problems. I also looked for the rogue files mentioned, and didn't find any.
Perhaps foolishly, on Wednesday afternoon I went back to speedtest.net, ran the test, and then checked again for virus & malware using MSE and Malwarebytes and nothing was found.
Maybe there a brief time when the speedtest.net site was compromised? Or it came from somewhere else?
Ian