First things first...
1) When posting, try to include a description of the problem in your post. Simply saying something doesn't work doesn't give people a lot to go on in terms of bug testings.
2) When posting, include a brief description of your probem in the title of your thread. Telling people what sort of problem you are having your title encourages people to come help, seeing a glib remark about how you are dumb doesn't really inspire people.
Now that that is over with, on to some code.
With that in mind here is your same code with the elements properly quoted:
You were making calls like
document.writeln('<DIV id="navText1">1 - 2</div>');
document.writeln('<DIV id="navText2">3 - 4</div>');
document.getElementById('navText1').style.visibility = "hidden";
document.getElementById('navText2').style.visibility = "hidden";
document.getElementById(x).style.visibility = "visible";
<a href="" onMouseOver = showLayer('navText1'); return true;>a</a>
<a href="" onMouseOver = showLayer('navText2'); return true;>b</a>
showLayer( navText1 )
PS. Indenting is your friend.