Try Sending the CB_SELECTSTRING Message to the Combobox, eg.
Code:
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, lParam As Any) As Long
Private Const CB_SELECTSTRING = &H14D

Private Sub Command1_Click()
    'lHwnd is the Handle of the Combobox you Found Previously..
    Call SendMessage(lHwnd, CB_SELECTSTRING, -1, ByVal "Guest")
End Sub
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Aaron Young
Analyst Programmer
[email protected]
[email protected]