I have been checking into a now mouse but decided to take a peek at the Autohotkey forum this morning and found this. It's an old entry. I haven't tried it out yet, (because I'm on the notebook right now, not the desktop) but thought that others may have the same problem with a mouse that doesn't have a scroll wheel and may want to give it a try:

http://www.autohotkey.com/forum/viewtopic.php?t=4954