JavaScript problem - please help me - opening new browser windows doesn't work

i've tried to open a new browser window, b-ut it doesn't work. can anyone help me? this is my code:

<script language="JavaScript">
var winTwo = null
function openNav() {
winTwo = window.open("http://www.myurl.com","Heading","width=600,height=400")
self.name = "fabr"
winTwo.blur()
}
function closeNav() {
if (winTwo != null) {
winTwo.close()
}
}
</script>

the openNav() en closeNav() are triggered by buttons (onClick="openNav()").

can anyone help me?

----------
Thanks a lot,
Leech.