ASP.NET

 <StartNavigationTemplate>
    <asp:UpdatePanel ID="upnlStartButton" runat="server" UpdateMode="Conditional" >
       <ContentTemplate>
          <asp:Button ID="StartNextButton" CommandName="MoveNext" runat="server" AlternateText="Start Writing Letter" Enabled="false" Text="Start Your Letter" />
       </ContentTemplate>
    </asp:UpdatePanel>
 </StartNavigationTemplate>


<WizardSteps>
    <asp:WizardStep runat="server" Title="Terms and Conditions" StepType="Start">
       &nbsp;

       <div style="margin-left: 10px; overflow: auto; width: 559px; height: 350px">
          <asp:Label ID="TermsAndConditionsLBL" runat="server"></asp:Label>

          <br />

          <asp:UpdatePanel ID="UpdatePanel1" runat="server">
             <ContentTemplate>
                <asp:CheckBox ID="agreeTermsCHBX" runat="server" Text="I Agree" AutoPostBack="True" OnCheckedChanged="agreeTermsCBX_CheckedChanged" />

                <br />

                <asp:CheckBox ID="disagreeTermsCHBX" runat="server" Text="I Disagree" AutoPostBack="True" OnCheckedChanged="disagreeTermsCBX_CheckedChanged" />
             </ContentTemplate>
          </asp:UpdatePanel>
       </div>
    </asp:WizardStep>

protected void agreeTermsCBX_CheckedChanged(object sender, EventArgs e)
{
  if (agreeTermsCHBX.Checked)
  {
     disagreeTermsCHBX.Checked = false;

     // enable button (Start Your Letter) to move to next page
     ((Button)findControl(wizard, "StartNextButton")).Enabled = true;

     // update button's panel
     ((UpdatePanel)findControl(wizard, "upnlStartButton")).Update();
  }
}