js：
function chkClick() {
    if (document.getElementById("chkBox").checked == true) {
        document.getElementById("button按钮的ID").disabled = false;                
        eturn true;
        } else {
            document.getElementById("button按钮的ID").disabled = true;                
            return false;
          }
     }    
<input type="checkbox" id="chkBox" class="checkbox_class" onclick="chkClick()" /><span class="textcolor2">《服务条款》</span></li>

<asp:Button ID="btnRegis" Enabled="false" runat="server" CssClass="login_btn" Text="立即注册" OnClick="btnRegis_Click" />