Lots of ways of doing this, depending on how you set things up. You're likely going to want to take advantage of the fact that you can store two pieces of information in your dropdownlist items. Here's one way:
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
If Not IsPostBack Then
Dim dt As New DataTable
dt.Columns.Add("Text")
dt.Columns.Add("Data")
Dim dr As DataRow = dt.NewRow
dt.Rows.Add(New Object() {Nothing, Nothing})
dt.Rows.Add(New Object() {"odd", "Checkbox1~Checkbox3~Checkbox5"})
dt.Rows.Add(New Object() {"even", "Checkbox2~Checkbox4"})
DropDownList1.DataValueField = "Data"
DropDownList1.DataTextField = "Text"
DropDownList1.DataSource = dt
DropDownList1.DataBind()
End If
End Sub
Private Sub DropDownList1_SelectedIndexChanged(sender As Object, e As System.EventArgs) Handles DropDownList1.SelectedIndexChanged
If DropDownList1.SelectedValue.ToString.Length > 0 Then
Dim chks() As String = DropDownList1.SelectedValue.ToString.Split("~")
For i As Integer = 0 To chks.Length - 1
DirectCast(Me.FindControl(chks(i)), CheckBox).Checked = True
Next
Else
For i As Integer = 1 To 5
DirectCast(Me.FindControl("Checkbox" & i), CheckBox).Checked = False
Next
End If
End Sub