place the particular DoCmd.Open code in the On_Change event of the combo box... use the current selection of the combo box to determine what exactly you need to open
Private Sub Combo0_Change()
Select Case Me.Combo0
Case 1
Do this...
DoCmd.OpenForm "myFormName"
DoCmd.OpenForm "myOtherFormName"
Case 2
DoCmd.OpenReport "thisReport",acViewPreview
DoCmd.OpenReport "thisOtherReport",acViewPreview
.
.
.
Case Else
DoCmd.HokeyPokey
DoCmd.TurnAround
End Select
End Sub