AVG "Trojan"

I used to follow virus technology in the middle nineties.  At that
time, there were three types of malware.  A "virus" attached itself to
an existing program which would still work correctly until it was time
to deliver the payload.  A "worm" found a way to launch itself without
using an existing executable.  And a "trojan" required the user to
download it and run it.

OK, today AVG announced that it found "Trojan Horse Generic10.crh" in
a printer install executable that's been on my computer for two years.
It hasn't been touched since I bought the printer in April 06.  How
could an existing file become a "trojan" now?

AVG then "repaired" the infection by deleting the file, so I cannot
compare to backups or anything.  I thought files were sequestered, not
just deleted. I'm not even sure I can find the install disks for that
printer.  

I tried googling for "Generic10.crh" and found three hits.  All three
were posts on the forums at AVG, and in all three cases the only reply
was that the posts were in the wrong forum.  One of those other posts
also involved a suspected false positive on a Brother printer driver.
-- 
Tomorrow is today already.
Greg Goss, 1989-01-27