You can leave the
Code:
Private Sub Form_Load()
    ValidStr = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
End Sub
in the Form_Load and use those changes in your Comand1_Click:
Code:
Private Sub Command1_Click()
Dim i As Integer
Dim Check As String
For i = 1 To Len (ValidStr)
   Check = Mid (ValidStr, i,1)
   If InStr(ValidStr, Check) > 0 Then
      MsgBox "Hello"
      Exit Sub
   End If
Next i
End Sub