Here is a code snippet.

code:
			$lblServiceType = $frmServiceDetail.Label()
			$lblServiceType.Text = "Service Type"
			$lblServiceType.Location = $iMargin, $lblStartName.Bottom + $iMargin
			$lblServiceType.Size = $ilblMediumWidth, $iHeight
			$cboServiceType = $frmServiceDetail.ComboBox()
			$cboServiceType.Location = $lblServiceType.Right + $iMargin, $lblServiceType.Top
			$cboServiceType.Size = $itxtLargeWidth, $iHeight
			$cboServiceType.Enabled = 0
			$cboServiceType.Style = 1
			$Nul = $cboServiceType.AddItem("Own Process")
			$Nul = $cboServiceType.AddItem("Share Process")
			$Nul = $cboServiceType.AddItem("Interactive Process")
			$Nul = $cboServiceType.AddItem("Kernel Driver")
			$Nul = $cboServiceType.AddItem("File System Driver")
			$Nul = $cboServiceType.AddItem("Adapter")
			$Nul = $cboServiceType.AddItem("Recognizer Driver")
			FOR $Index = 0 TO $cboServiceType.ListCount - 1
				$cboServiceType.ListIndex = $Index
				IF $cboServiceType.Value = $objService.ServiceType
					$Index = $cboServiceType.ListCount
				ENDIF
			NEXT
If I underestand the question, this is the part that will hopefully help. I use it to find the active value, but you could also use it for excluding aready entered values.
code:
			FOR $Index = 0 TO $cboServiceType.ListCount - 1
				$cboServiceType.ListIndex = $Index
				IF $cboServiceType.Value = $objService.ServiceType
					$Index = $cboServiceType.ListCount
				ENDIF
			NEXT
_________________________
Kelly