var banner=new Array()
banner[0]='video_linki'
banner[1]='video_linki''
banner[2]='video_linki''
banner[3]='video_linki''
banner[4]='video_linki''
var hangisi=Math.floor(Math.random()*(banner.length))
document.write(banner[hangisi])

<script language="JavaScript" src="random.js"></script>