' Watch for Delete keys.
Private Sub txtInput_KeyDown(KeyCode As Integer, Shift As _
    Integer)
    If KeyCode = vbKeyDelete Then CheckText KeyCode, True
End Sub

' Validate key presses.
Private Sub txtInput_KeyPress(KeyAscii As Integer)
    CheckText KeyAscii, False
End Sub

' Set KeyValue = 0 if the result of this key press
' would be invalid.
Private Sub CheckText(ByRef KeyValue As Integer, ByVal _
    is_delete As Boolean)
    If Not ValidChange(KeyValue, is_delete) _
        Then KeyValue = 0
End Sub

' See if this change is valid.
Private Function ValidChange(ByVal KeyValue As Integer, _
    ByVal is_delete As Boolean) As Boolean
Dim str_value As String
Dim is_valid As Boolean
Dim int_value As Integer

    ' See what the new value will be.
    str_value = NewValue(KeyValue, is_delete)

    ' See if the new value is valid.
    If Len(str_value) = 0 Then
        ' The text is blank. Allow it.
        is_valid = True
    ElseIf str_value = "-" Then
        ' The text is "-". It might be part of
        ' a negative integer. Allow it if
        ' m_MinValue < 0.
        is_valid = (m_MinValue < 0)
    Else
        ' See if the text is an integer.
        On Error Resume Next
        int_value = CInt(str_value)
        If Err.Number <> 0 Then
            ' The text is not an integer.
            is_valid = False
        Else
            ' See if the value is within bounds.
            is_valid = _
                (int_value >= m_MinValue) And _
                (int_value <= m_MaxValue)
        End If
        On Error GoTo 0
    End If

    ' Return the result.
    ValidChange = is_valid
End Function

' See what the new value would be if this change
' is allowed.
Private Function NewValue(ByVal KeyValue As Integer, ByVal _
    is_delete As Boolean) As String
' Useful ASCII codes.
Const ASC_CTRL_V = 22
Const ASC_CTRL_X = 24
Const FIRST_ASC = 32    ' 1st visible char.
Const LAST_ASC = 126    ' Last visible char.

Dim old_txt As String
Dim part1 As String
Dim part2 As String
Dim part3 As String

    old_txt = txtInput.Text

    ' Find the pieces before and after the section.
    part1 = Left$(old_txt, txtInput.SelStart)
    part3 = Right$(old_txt, Len(old_txt) - _
        txtInput.SelStart - txtInput.SelLength)

    ' Calculate the new text.
    part2 = ""
    If is_delete Then
        ' This is a delete character.
        ' If no text is selected, delete the
        ' following character (if there is one).
        If (txtInput.SelLength <= 0) And _
           (Len(part3) > 0) _
        Then
            part3 = Mid$(part3, 2)
        End If

        ' If text is selected or part3 is blank,
        ' just leave part2 = "".
    Else
        ' This is not a delete character.
        ' See what it is.
        Select Case KeyValue
            Case vbKeyBack      ' Backspace.
                ' If no text is selected, delete the
                ' previous character (if there is one).
                If (txtInput.SelLength <= 0) And _
                   (Len(part1) > 0) _
                Then
                    part1 = Left$(part1, Len(part1) - 1)
                End If
                ' If text is selected or part1 is
                ' blank, leave part2 = "".

            Case ASC_CTRL_V
                ' Paste the clipboard's text over
                ' the selected text.
                part2 = Clipboard.GetText(vbCFText)

            Case ASC_CTRL_X
                ' Leave part2 = "".

            Case FIRST_ASC To LAST_ASC
                ' Use the visible character typed
                ' for the middle string.
                part2 = Chr$(KeyValue)

            Case Else
                ' Assume other non-visible keys
                ' like ^C will not change the text.
                NewValue = old_txt
                Exit Function
        End Select
    End If

    ' Build the result.
    NewValue = part1 & part2 & part3
End Function

You can use similar techniques to perform other "after the fact" validations on fields.