JavaScript not working on browsers
===

I have this JS function to check that at least one checkbox group is selected:

    function chkChecks()
    {
    	event.preventDefault();
    	isChecked=false
    	for(var i=0;i<document.forms["form1"]["SearchEngines[]"].length;i++)
    	{		
    		if(document.forms["form1"]["SearchEngines[]"][i].checked)
    		{
    			isChecked=true
    		}	
    	}
    
    	if(isChecked)
    	{
    		document.forms["form1"].submit()
    	}
    	else
    	{
    		alert('Please select at least one Search Engine')
    		return false;
    	}
    
    }
    </script>

The form itself looks like this:

    <form id="form1" name="form1" method="get" action="main_results.php">
          <input name="q" type="text" size="67"/>
          <input class="submit" type="submit" value="Search" onclick="chkChecks()" />

This works fine for chrome but not in IE or Firefox, I can't figure it out.

In the other two browsers, the form is sent as if there was no problem