This comes up on a regular basis.
Even with a calibrated monitor a lot of canon printers output darker that the screen, make sure you are running the correct paper profile firstly, then in the printer driver turn down the colour intensity, usually by about -5 to -8 see if that helps, also change the colour mode to linear tone.
If you are using cs2 or cs3 iirc there is a utility that you print 2 pages out that give various adjustments, you then match these to the screen and adjust the driver setting to match.