Automatic Population of Objects on a Web Page

I have a web page (main menu) which contains a dropdownlist which lists the agent numbers that are available. This list is generated via a stored procedure which returns a list of agents and the ASP code then creates the list. When I go into that web page for the first time the dropdown list shows the first agent number in the list and code in the Page_Load event processes that agent number to create some information I need on other web pages. If I select another agent number from the list, code in the SelectedIndexChanged event for the dropdownlist, which is similar to code in the Page_Load event, does some processing and generates a list of values that I need for use on other web pages. This works fine.
The problem arises when I enter the application the second time. Somehow the application/system knows the last value that was selected in the dropdown list and displays that value in the dropdown list. However, in the Page_ ...