You can use AJAX to acheive this.
1) Use webservice and the webmethod of that webservice will return in form of xml data that is queried from database.
2) On keypress event of the textbox you will call the webservice using java script.
3) Use java script to fill the listbox like control by the xml data parsed in list like format just below the textbox.
For more details you can refer
http://www.iansuttle.com/blog/post/Applying-AJAX-Using-ASPNET-11.aspx[
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