document.<form name>.<checkbox name>.checked returns true/false, so use this to determine IF checked. Or you can use getElementsByTagName[<index number of checkbox>].checked, or you can use getElementById[<ID of checkbox>].checked
Using getElementById you can then get the element whose text is to be changed and use .innerHTML on it to change the text. innerText should also work to capture the text node itself if you want to access the parent directly while there are other tags within that element.
You might also want to store the retrieved element in a variable and just access it with variable.checked, which would improve performance, though probably insignificantly, but if you're going to refer to that element several times it will definitely shorten the code.
Of course, you need to attach a 'click' event handler on the checkbox to execute the code. How you do this depends on the browser you're using, so you should probably look up event handlers in detail for your browser. The best thing to do is to use a library such as prototype or jQuery that handles the element checking for you and assigns according to what is available on the client. IE uses attachEvent(), W3C compliant browsers use addEventHandler(). Good news is both browsers simply accept .<event>, such as .******(), but this approach is being considered deprecated, so it's best to stop using it just to stay in gear.
[edit] the stars were meant to show on-click without the '-', it seems the forum is taking some precautions to avoid XSS attacks 🙂
