prendi spunto da qui

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
  <meta http-equiv="content-type" content="text/html; charset=windows-1250">
  <meta name="generator" content="PSPad editor, www.pspad.com">
  <title></title>
  </head>
  <body>
<input type="checkbox" name="pippo" value="ciao" onclick="document.getElementById('pluto').disabled=this.checked == false">
<input type="text" id="pluto" name="pluto" value="ciao" disabled="disabled">
  </body>
</html>