You can do the following to check and remove your old drivers.
Assuming your using Windows, click on START, then Printers and Faxes.
In the new window click on File and then Server Properties.
From here select the Drivers tab and see what drivers are listed there and select any Lexmark ones and push the Remove button. Reboot.
You may want to disconnect your Canon printer and remove it's drivers also and then reinstall them fresh again and see how you go.
You could also try something like Driver Genuis -
http://www.driver-soft.com/ which is supposed to be able to remove old and unused drivers, but it's a paid product and i'm not sure how effective it is.
I would tend to agree with the Dell staff that it should matter if you have old drivers still on the system as it will only use the drivers/files that are required by your hardware.
I would also try the different USB ports on your Dell machine and possibly a different USB cable just in case.