Disable button WITHOUT using onClick

Hi all,

I need to do the following:

When a button is clicked, the button then disables (as well as running its normal function).

Obviously I'm used to using this.disabled=true but in this instance I cannot access the onClick in the <input> tag because the page is hard coded by a third party application.

So I'm trying to make a function which locates the button, notices if it's been clicked and disables it on click. Seems easy to me! However I cannot make it work!

Here are the two variations I've tried (both call the function in the page onLoad):

function disableButton() {

document.getElementById('submit').onclick=document.getElementById('submit').disabled=true;

}

and....

function disableButton() {

if(document.getElementById('submit').onclick=true)

{

document.getElementById('submit').disabled=true;

}

}

Any ideas? Because this seems like it should be way easier than I'm finding it right now!

Thanks