Autofilling controlss

Question

hello, i got question, i need to make some mechanism which will be fillling windows form controls(c#) of  some data(user name).... for example,
i want to send mail to someone when i will press "a" in textbox,or dropdownlist i will see user which names start on "a",
any idea?
regards
ps. what is the best place to store the data with name of user only database or maybe some config file?