Choosing a Radio Button to effect a Text Box

Hello Friends, I would like to modify a form when a user select the 1st radio button the user is required to enter SSN/Acct, if they chose the 2nd button then the text box would be grayed out. 
 O   Button1
 O   Button2
[ this is the text box where they enter their number]
In the HTML the button is define as a RadioButtonList id.........
    <asp:ListItem Value = "Button One"> Button One</asp:ListItem
    <asp:ListItem Value = "Button Two"> Button Two</asp:ListItem