Using window.onLoad = or window.onLoad +=

I'm trying to avoid ViewState Mac errors and one of the solutions is to disable the form and re-enable it when the page loads to prevent premature submissions of the form before it has finished loading (see here for a good discussion of the options - http://blogs.msdn.com/b/tom/archive/2008/03/14/validation-of-viewstate-mac-failed-error.aspx)

To enable the form again the above page has this:

  function enableForm() { 
      document.getElementById("form").disabled = false; 
  } 

  window.onLoad = enableForm();

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I wasn't sure if this would replace any existing window.onLoad event handlers, and if you are supposed to enter

  window.onLoad += enableForm();

and if you are supposed to do this whether it would put "enableForm()" as the first function called after onLoad?