Hey, I have a form like this...

Blah []1 []2 []3 []4 []5 []6
Blah2 []1 []2 []3 []4 []5 []6

[]= Checkbox

Ok, So if they click on the "2" it would be checked, then if they click on the 5 it would uncheck the 2 and check the 5. How do I do this... and I need it to do the same for blah2 but not affect blah, if you know what I mean.

I understand it would be easy to use drop down boxes, but that's not the way my client wants it.

Use Java Script ;)
Add onClick event for each checkbox and process the events by JS.

Use radio buttons. Make sure all radio buttons in the group have the same name attribute, but different value attribute.