savecopyas [@Dieter Klemke?)

Private Sub ComboBox1_Click()
Dim datei As String
Dim BereitsGeöffnet As Boolean
Dim wb As Workbook
If ComboBox1.ListIndex > 0 Then
datei = ComboBox1.Value
For Each wb In Workbooks
If wb.Name = datei Then
BereitsGeöffnet = True
Exit For
End If
Next wb
If BereitsGeöffnet Then
Workbooks(datei).Activate
Else
Workbooks.Open Filename:=ordner & datei
End If
End If
End Sub

Private Sub UserForm_Initialize()
Dim fil As File
Dim fol As Folder
Dim fso As FileSystemObject
ordner = "D:\VBA_Test\"
Set fso = New FileSystemObject
Set fol = fso.GetFolder(FolderPath:=ordner)
With Me.ComboBox1
.AddItem ""
For Each fil In fol.Files
If Right$(fil.Name, 4) = ".xls" Then
Me.ComboBox1.AddItem fil.Name
End If
Next fil
.ListIndex = 0
End With
End Sub