Вы делаете это неправильно!
<!DOCTYPE html>
<style>
input[type="button"]{background: #FFFF00;}
input[type="button"]:hover {background: #00FF00;}
input[type="button"]:disabled {background: #fff;}
</style>
<form>
<input type="checkbox" value="asd" id="tb1" onclick="form.tbnum.disabled = !this.checked">
<input disabled="" type="button" value="Продолжить" name="tbnum" onclick="document.location='anketa.html'" />
</form>