Field Change Question

HI need some help!

I have a field (Category) that is a drop down box with category names. (Kitchen, Bedroom, Garage, Garden)
I have a hidden number field (Category_Code) which has category codes (001, 002, 003, 004)

I would like the hidden field to be updated with the correct code based on the choice selected in the drop down category name box. So choose bedroom and category code will = 002

Can this be done with script?

Martin