<script language="javascript">
   function check(){
       if(!document.getElementById("a").checked){//没有选中
       document.getElementById('submit').style.backgroundColor="#666666"
   document.getElementById('submit').style.borderColor="#333333"
   document.getElementById('submit').style.color="#000000"
       document.getElementById('submit').disabled="disabled";  
   }else{//选中
       document.getElementById('submit').disabled=""; 
   document.getElementById('submit').style.backgroundColor="#0033FF"
   document.getElementById('submit').style.borderColor="#0066FF"
   document.getElementById('submit').style.color="#ffffff"
   } 
   }
</script><form name="f">
 <INPUT TYPE="checkbox" id="a" onclick="check()" checked="checked"> 
 <INPUT TYPE="submit" VALUE="注册" id="submit"> 
</form>