You can also use the "isnumeric" function in the following way:
If IsNumeric(TextBox1.Text) Then
MsgBox "OK"
Else
MsgBox "no good"
End If

You can limit entry to maximum one digit by changing the "maxlength" property
of your textbox to 1. You can access the properties menu in the VBEditor. If it
is not already displayed press the F4 key. Be sure that you change the property
of your textbox1 control.