Silly question here, but have you tried the same setup at home that you have at work? The DIN -> PS/2 -> USB route?
The only time I've heard the PC system speaker go off while using a keyboard is when pressing more keys then a board can handle(rollover problems). In those cases though, the key that triggers the problem does not register at all, so it doesn't really sound like what you've got going on. :noidea: