datagrid/listbox question

I need to create a listbox control that can be used in edit mode that highlights previously selected items from it's table. Here is the scenario: Table 1 - contains employee benefit information. Table 2 - contains employee information (lname, firstname etc.) Table 3 - contains types of benefits Table 4 - contains composite information from both Table 2 and Table 3. For example, what employees selected which benefits. This table would contain fkey id's from Table1 and Table2. What I need to do is discover how to use a listbox control to both display the contents for Table 4 and then use the same control in edit mode to select additional items from Table 3 with those previously selected highlighted. Can someone tell me how to do this? Thank you. Code in C# please. This is a webform application. Sincerely, Tim