Fill up text box from value selected in combobox

I am working with MS Access 2007. I have a table that contains student names corresponding to student IDs. I have student IDs in another relationship table and a form that has a combobox that contains all the student IDs from the relationship table. What I want to be able to do is, upon selection of a student ID from the combobox, another text box in the same form will be automatically filled up with the student name by looking up the student table. I have been trying to achieve this since last night so any help will be highly appreciated. Thanks