There was a recent update to Filezilla, which I use for transferring image files to Alamy. I made a mistake and ended up with a server update instead of client, so I uninstalled, re-downloaded, and re-installed Filezilla client. Apparently the Filezilla download contained a virus which caused BING to become my default homepage. I say only "apparently" because I have no hard proof, but the Filezilla download was the only change I made to the computer.
This is just a heads-up in case others run into this problem.
--Michael