Use the keypress method on the field in qustion to run the following (the second if statement should be modified if you don't want the user to enter "," or "." characters)

Code:

If KeyAscii< Asc("0") Or KeyAscii> Asc("9") Then
    If KeyAscii = Asc(",") Or KeyAscii = Asc(".") Or KeyAscii = 8 Then
      'nothing, we will allow these characters
    Else 'the user has pressed some key other than 0-9 or "," or "." or backspace
        KeyAscii = 0 ' cancel the character
        field.SelStart = 0
        field.SelLength = Len(field)
        Beep
    End If
End If