This is a very easy to solve problem. There is actually a html tag which does exactly that. It's called the <noscript> tag. The noscript tag is used to define alternate content, usually text, when a script does not execute. Please note however, if your script does not run, but the browser supports javascript, then the text in the <noscript> tag will not display either.
To use the <noscript> element, place it after your <script> tag.

<script type="text/javascript"> 
<!--
document.write("Hello World!")
//--> 
</script> 
<noscript> Your browser does not support JavaScript!</noscript> 

Remember, you should wrap the script in a HTML comment, because older browsers which do not recognize the <script> tag would display the code on the page, but because of the HTML comment they would take the script and place it inside a HTML comment, so it will not actually display.