To make sure that the type of data in the Clipboard is or not a numeric value. Test this:

    If Val(Clipboard.GetText) = 0 Then Clipboard.Clear

And then, if you don't wanna loose the data of the Clipboard, first copy it to a variable and then give it back to the clipboard. The full code is liisted here:

Dim clp As String

Private Sub t_MouseDown(Button As Integer, Shift As _
    Integer, X As Single, Y As Single)
    RetainClp
End Sub

Private Sub t_KeyPress(KeyAscii As Integer)
    If KeyAscii = 8 Then Exit Sub
    If KeyAscii < 48 Or KeyAscii > 57 Then KeyAscii = 0
End Sub

Private Sub t_LostFocus()
    If Len(clp) > 0 Then Clipboard.SetText clp
End Sub

Sub RetainClp()
    If Val(Clipboard.GetText) = 0 Then
        clp = Clipboard.GetText
        Clipboard.Clear
    End If
End Sub