With the function below you can determine if a file is in use by another process. The function returns  True if you can't get full access to the file.

Function FileAlreadyOpen(FullFileName As String) As Boolean
' returns True if FullFileName is currently in use by another process
' example: If FileAlreadyOpen("C:\FolderName\FileName.xls") Then...
Dim f As Integer
    f = FreeFile
    On Error Resume Next
    Open FullFileName For Binary Access Read Write Lock Read Write As #f
    Close #f
    ' If an error occurs, the document is currently open.
    If Err.Number <> 0 Then
        FileAlreadyOpen = True
        Err.Clear
        'MsgBox "Error #" & Str(Err.Number) & " - " & Err.Description
    Else
        FileAlreadyOpen = False
    End If
    On Error GoTo 0
End Function