Combobox and Database.

Hello All,
I have a problem, which is related to database(MSAcess) and Combobox.
i have A field named UserID (now There are 8 fields) in the Database with table name as Registration. in my design i have a combobox with ineed to populate with count of UserID in such a way that when i select 1, Only first userID should be Selecetd, when i Select 2 from Combobox ,first two userid should be selected......its abig worry for me to achive..plz help...i hav tried all option upto my concern..its Urgent.
Thanks in advance