drop-down boxes for Excel

open excel
click on help
type drop in the search field
read

seriously, there's a ****load of options and possibilities depending on how you want to do it (and yes, all in the help file)

sounds like a database type of application better than excel

Put your list of customers in a column off to the right of your "form".

In the first cell where you want a dropdown, hit Data-Validation. Choose List. If your customer names are in cells K2 through K252, you can type:

=$K$2:$K$252

into the box, or you can click the icon at the right end of the box and select the list with the mouse. Once the list cells are selected, hit the icon again.

Hit Ok.
You'll need to copy this cell down as far as you want checkboxes to go.
This is the easiest way to use a dropdown box in Excel. There are two other ways.