Two things can cause gibberish print. Incorrect/corrupt print driver, and sending data to the printer faster than it can buffer/print it.
In the first case, make sure the driver is correct, the most recent available, and reinstall it to be sure it's not corrupt.
In the second case, if you are using parallel connection, some printers are not compatible with "ECP" mode that is used by default in most versions of Windows. You can try changing the CMOS setting "Parallel port mode" from ECP to SPP or NORMAL to see if that helps.
One other thing that I have seen in my job (supporting users of a major printer corporation) is corrupt print jobs, especially in Word. If a document is edited too many times, it could be corrupt, try recreating it, or create a new document to see if it still prints gibberish.