You cna write code or an easier way is to use radiobuttons and change their appearance to look like checkboxes. In the Object Paltte under the field tab there is an appearance dropdown. Change it to a solid square and voila ....mutually exclusive checkboxes.
Hope that helps
Thank You Daniel this worked perfectly. I know you can use radio buttons to this but the form I was working on was very old and contained subforms within subforms and it was just a mess. When I use the radio buttons it just doesn't join them in a group so I had to use checkboxes and this code works exactly fine with the click function. Thanks again.