You can do that "as you type" and/or on LostFocus evnt:
VB Code:
  1. Private Sub Text1_KeyPress(KeyAscii As Integer)
  2.     KeyAscii = Asc(LCase(Chr(KeyAscii)))
  3. End Sub
  4.  
  5. Private Sub Text1_LostFocus()
  6.     Text1.Text = LCase(Text1.Text)
  7. End Sub