Enable checkbox only if fields are correctly filled [on hold]

I want that my check box should be enable only when all the fields are filled correctly and should not be empty.

This is registration form with JavaScript validation. I just want that the check box should be enable when all the fields are correctly filled.

<form name='frm'>
  Firstname:<input type="text" name='fname' id="firstname" maxlength="50" onblur="validateformfirst()" />
  <div id='firsterror' style="color:red"></div><BR>

  Lastname:<input type="text" name='lname' id="lastname" maxlength="50" onblur="validateformlast()"/>
  <div id='lasterror' style="color:red" ></div><br>

  Email:<input type="text" name='email' id="email" maxlength="200" onblur="validateformemail()"/>
  <div id='emailerror' style="color:red"></div><br>

  Password:<input type="password" name='password' id="password" maxlength="100" onblur="validateformpassword()" onKeyUp="verify.check()"/>
  <div id='passworderror' style="color:red"></div><br>

  Confirm Password <input type="password" name='confirmpassword' id="confirmpassword"  onKeyUp="verify.check()"/>
  <div id='confirmpassworderror' style="color:red"></div><br>

  <input type="checkbox"  name="terms" id="signin"  onClick="EnableRegister(this)" DISABLED="disable"/><span>I read Terms and Conditions</span> 

  <button type="submit" onclick=" validateForm()" id="Register" >Register</button>
</form>

and this is my javascript code which validate my form....can anyone combine this validation in such a way that...once all validate data is filled in the text box,,the checkbox should be enable...else it should be disable untill all filled correctly.

<script>
function validateformfirst(fname)
{
   var str = true;
    var charfilter = /^[a-zA-Z]*$/;
if (document.frm.fname.value == "") {
        document.getElementById("firsterror").innerHTML = "Please enter FirstName";
        str = false;
    } else if (!charfilter.test(document.frm.fname.value)) {
        document.getElementById("firsterror").innerHTML = "Please enter valid First Name";
        str = false; 
    }
    else{document.getElementById("firsterror").innerHTML = "";}
    return str;
}

    function validateformlast(lname) 
    {
    var str = true;

    var charfilter = /^[a-zA-Z]*$/;
    if (document.frm.lname.value == "") {
        document.getElementById("lasterror").innerHTML = "Please enter LastName";
        str = false;
    } else if (!charfilter.test(document.frm.lname.value)) {
        document.getElementById("lasterror").innerHTML = "Please enter valid last Name";
        str = false;
    }
    else{document.getElementById("lasterror").innerHTML = "";}
    return str;
    }

    function validateformemail(email) 
    { 
    var str = true;

    var mailformat = /^\w+([\.-]?\w+)*@\w+([\.-]?\w+)*(\.\w{2,3})+$/;  
    if (document.frm.email.value == "") {
        document.getElementById("emailerror").innerHTML = "Please enter email";
        str = false;
    } else if (!mailformat.test(document.frm.email.value)) {
        document.getElementById("emailerror").innerHTML = "Please enter valid email";
        str = false;
    }
    else{document.getElementById("emailerror").innerHTML = "";}
    return str;
    }



    function validateformpassword(password)
  {
   var str = true;
   var passwordformat=/^(?=.*?[A-Z])(?=.*?[a-z])(?=.*?[0-9])(?=.*?[#?!@$%^&*-_]).{8,}$/;
   if (document.frm.password.value == "") 
   {
        document.getElementById("passworderror").innerHTML = "Please enter password";
        str = false;
    } else if (!passwordformat.test(document.frm.password.value)) 
    {
        document.getElementById("passworderror").innerHTML = "Please enter valid password";
        str = false;
    }
    else{document.getElementById("passworderror").innerHTML = "";}
    return str;
    }
</script>