If this is a microsoft mouse and you're using the intellipoint driver do the following:
With ArchiCAD running go to Start->Control Panel->Mouse. Go to the Wheel tab (I think) and click the Troubleshoot(er) button. It will ask you to identify the program with the problem. Select ArchiCAD from the list of running programs and continue the troubleshooter. Once finished this should fix the problem. You'll need to repeat the same process with PlotMaker.
Cheers,