Two Level Combo:Fill Second Combo Depending On First Combo Value

Hi Experts,
i am developing a page in which i have filled COMBO1 with Products and COMBO 2 with its sizes(based on the selection made in combo 1)..
i used the following code:
FOR COMBO 1:
    Private Sub GetProduct()
        Dim AdminCmd As SqlCommand
        Dim AdminReader As SqlDataReader
        con = New SqlConnection(ConfigurationSettings.AppSettings("strConnect"))
        con.Open()
        AdminCmd = New SqlCommand("EXEC stp_PROD_GetProducts", con)
        AdminReader = AdminCmd.ExecuteReader
        cmbProduct.DataSource = AdminReader
        cmbProduct.DataValueField = "ProductID"
        cmbProduct.DataTextField = "Product"
        cmbProduct.DataBind()
        con.Close()
    End Sub

FOR COMBO2:
    Private Sub GetSizeCategoryByProductID()
        Dim AdminCmd As SqlCommand
        Dim AdminReader As SqlDataReader
        con = New SqlConnection(ConfigurationSettings.AppSettings("strConnect"))
        con.Open()
        AdminCmd = New SqlCommand("EXEC stp_SIZE_GetSizeCategoriesByProductID " & cmbProduct.SelectedItem.Value, con)
        AdminReader = AdminCmd.ExecuteReader
        cmbSizeCategory.DataSource = AdminReader
        cmbSizeCategory.DataValueField = "SizeCategoryID"
        cmbSizeCategory.DataTextField = "SizeCategory"
        cmbSizeCategory.DataBind()
        con.Close()
    End Sub

The problem is that the code is working but it submits page everytime i made selection in COMBO1.i want to fill second combo without submitting the page..is there any javascript or anyother way to do this..i have done this in ASP using javascript but can any1 guide or provide me to do this in asp.net on client side.i dont want to submit the page
i hope u guys got the point..