hola, proba con esto:
Private Sub TextBox1_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles TextBox1.KeyPress
If (Not e.KeyChar Like "[a-z]") And (Asc(e.KeyChar) < 8 Or Asc(e.KeyChar) > 32) Then
MsgBox("Ingrese únicamente letras minusculas.", MsgBoxStyle.Exclamation)
e.Handled = Not (e.KeyChar = "0")
End If
End Sub
o de esta otra manera q es igual
Private Sub TextBox1_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles TextBox1.KeyPress
If (Asc(e.KeyChar) < 97 Or Asc(e.KeyChar) > 122) And (Asc(e.KeyChar) < 8 Or Asc(e.KeyChar) > 32) Then
MsgBox("Ingrese únicamente letras minusculas.", MsgBoxStyle.Exclamation)
e.Handled = Not (e.KeyChar = "0")
End If
End Sub
Saludos.