The problem is most likely the print driver or the AppleTalk driver.
Upgrading to 7.1P2 is worth a try if the suggestions below don't help.
HP provided some helpful information on where the "garbage characters"
might be coming from:
* HP ships two drivers with the DeskWriter C printer: one for an
AppleTalk connection, the other for a direct serial connection. If the
HP DeskWriter serial driver is being used and AppleTalk is (or was
previously) ON, there is a good possibility that some "AppleTalk data"
was sent to the printer buffer and printed as "garbage". One way to
clean the print buffer completely is to power cycle the printer.
* In rare instances, spurious data is sent to the printer if the
printer is already powered on when the Macintosh is turned on. Try
starting up the Macintosh on first, then powering on the DeskWriter.
* Be sure the serial cable you're using is an HP cable #92215S or Apple
cable #M0197 for a direct serial connection.