' Declarations:
Private Declare Function CopyFile Lib "kernel32.dll" Alias "CopyFileA" _
    (ByVal lpExistingFileName As String, ByVal lpNewFileName As String, _
    ByVal bFailIfExists As Long) As Long

' Code:
Private Sub cmdCopyFile_Click()
    Dim retval As Long
    retval = CopyFile("C:\SourceFile.txt", "C:\DestinationFile.txt", 1)
    If retval = 0 Then ' failure
        MsgBox "Copy failed -- C:\DestinationFile.txt already exists."
    Else ' success
        MsgBox "Copy succeeded."
    End If
End Sub