I'm sure there's an easier way, but try:

Private Sub TextBox1_Change()
Static REX As Object
If REX Is Nothing Then
            Set REX = CreateObject("VBScript.RegExp")
            With REX
        .Global = True
        .Pattern = "[^0-9]"
            End With
    End If
TextBox1.Value = REX.Replace(TextBox1.Value, vbNullString)
End Sub
Hope that helps,

Mark