Private Sub Timer1_Timer()
    enemy1.Left = enemy1.Left - 5
End Sub

Private Sub Timer1_Timer()
    Do
       enemy1.Left = enemy1.Left - 5
       If enemy1.Left = < 5 Then
          Do
             enemy1.Right = enemy1.Right + 5
          Loop Until enemy1.Right = > 1000 'or whatever your size is
       End If
    Loop
End Sub

Option Explicit
Const nTwipsPerMove = 10
Private Sub Timer1_Timer()
    Static strDirection As String
    If strDirection = "" Then strDirection = "left"
    If enemy.Left > 0 Then
        If strDirection = "left" Then
            enemy.Left = enemy.Left - nTwipsPerMove
        ElseIf strDirection = "right" Then
            enemy.Left = enemy.Left + nTwipsPerMove
        End If
    End If
    If enemy.Left = 0 Then
        If strDirection = "left" Then strDirection = "right"
        enemy.Left = enemy.Left + nTwipsPerMove
    End If

    If enemy.Left + enemy.Width = Me.Width - nTwipsPerMove * 5 Then
        If strDirection = "right" Then strDirection = "left"
        enemy.Left = enemy.Left - nTwipsPerMove
    End If
End Sub