check boxes

[Sibes]

I am doing a asset database. I am stuck when it comes the office machines.
I have a combo box with 5 items.
Printer,
Scanners
Copiers,
Fax,
Mulitpurpose.

I also have 4 checkboxes
Printer,
Scanner,
Copier,
Fax

When Multipurpose is selected, I need the user to select at 2 items from 4
checkboxes.

I have it so that one must be checked, but i cant figure out how to ensure
that 2 or more fields are selected to describe the multipuprose machine.

Any suggestions??

Thank you all for your help ..