Adobe's instructions are to remove Adobe Speed Launch from All Programs/Startup, but there was nothing there. It was present in start/run/msconfig's Startup tab. After searching the files for any mention of reader_sl, checking for options within Adobe Reader, looking in Reader's Help and the Adobe website, I finally checked the Registry.
I made a full export of the registry, then a branch export of \HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run.
I then deleted the single entry mentioning reader_sl from that branch.
Presto, no longer loading at startup, and now gone from msconfig's Startup tab. The machine is much friskier. After a boot, My Computer takes only 4 seconds to load.