hamluis and
keyboardNinja, I took your advice and unchecked the blank startup entry. I got a message from the computer saying I had to log on as an administrator to make such a change. (I thought that was odd since this my home computer and there is only the one account on it. I've always assumed that account had administrator's rights.) At any rate, after unchecking the entry and restarting the computer the blank entry has disappeared.
keyboardNinja,
computerxpds, and
Andrew - you guys were right. The system tray icon was the Adobe updater. I feel kind of foolish now.

When three strange things (the virus, the blank start up entry, and the new icon) showed up at more or less the same time, I jumped to the conclusion they were related to each other.
Thanks for the help, everybody. If anything odd occurs I'll be back looking for your wisdom again.