Thanks, Mark - yes, that's the one.
Glad it's not an infection anyway. I wonder whether it's just some kind of incompatibility issue.
I didn't note the full details but this is what I did note from the boot scan:
SPLASHLITE 1.7.1 setup.exe/splash_install.msi|...CAB archive is corrupted (this was described as ERROR 42127)
[another SPLASHLITE 1.7.1 setup/install file]: OLE archive is corrupted (this was described as ERROR 42144)
[another SPLASHLITE 1.7.1 setup/install file]: CAB archive is corrupted (this was described as ERROR 42127)
Thanks for volume icon advice - I found that it was marked 'hide when inactive' - though I don't know how it makes its apparently-random decisions on when it is inactive! It returned again yesterday then was gone again this morning, and is still absent despite my changing the setting to 'Always Show' - maybe I need to reboot.
Everything else is fine after just deleting the download - the program works and doesn't cause problems with folders. It's amazing how things have speeded up since deleting the download - it would never have occurred to me that it was just an item in the Download folder slowing things down. I can't find any other free player that plays mts files - vlc won't do it for me.