Cry For Help! - Filling In Forms Automatically From Tables

I am DESPERATE for a quick reply.. have exhausted my options...

i have a table called order details which contains:

order id
product id
product name
quantity
unit price
sale price
discount

and a products table containing:
Product id
product name
unit price
units in stock

I have a form which currently shows text boxes for
order id
product id
product name
quantity
unit price

(product id is a combo box)

i can choose the product id from the drop down i have created but it does
not put any other data into any of the other fields.. how on earth EXACTLY
can i manage to do this? i want the data from the product table to fill in
the form once i choose the product id from the form combo box

cheers for ANY help