See inline comments...

Whatever code you are showing your UserForm from, use this statement to show
it instead of the statement you have now (assuming your UserForm is still
named UserForm1; if not, use your own UserForm's name instead... the key is
the vbModeless argument)...

UserForm1.Show vbModeless

Give this new code a try...

Private Sub CommandButton1_Click()
 Dim X As Long
 Dim LastColumn As Long
 Dim Number1 As Variant
 Dim Number2 As Variant
 Dim TBox1 As String
 Dim TBox2 As String
 TBox1 = Trim(TextBox1.Text)
 TBox2 = Trim(TextBox2.Text)
 If TBox1 = "" Or TBox2 = "" Then
   MsgBox "You must fill in both text boxes!"
 ElseIf TBox1 Like String(Len(TBox1), "#") And Len(TBox2) < 29 Then
   Number1 = CDec(TBox1)
   If TBox2 Like String(Len(TBox2), "#") And Len(TBox2) < 29 Then
     Number2 = CDec(TBox2)
     If Number2 < Number1 Then
       MsgBox "TextBox2 must contain a larger number than TextBox1"
     Else
       LastColumn = Cells(1, Columns.Count).End(xlToLeft).Column
       If LastColumn = 1 And Range("A1").Value = "" Then LastColumn = 0
       For X = 0 To Number2 - Number1
         Cells(X + 1, LastColumn + 1).Value = _
                "'" & Format$(Number1 + X, String(Len(Trim(TBox1)), "0"))
       Next
     End If
   Else
     MsgBox "Bad entry in TextBox2"
   End If
 Else
   MsgBox "Bad entry in TextBox1"
 End If
End Sub

Rick