Excel Combo Box

I have designed a Purchase Order using Excel 2000. The form contains a combo box with five selections listed. When I protect the Form the combo box will not allow me to select the item I wish to post to the form. I have reviewed the Protection Lock for each of the cells in the form and have locked the cells that are required to be locked. I have unchecked the locked Combo Box so that it is unlocked but I can not select the item. If I leave the form unprotected I can use the Combo Box without a problem. Any assistance would be appreciated.