наверное так:
<HTML>
<HEAD>
<TITLE>Untitled Document</TITLE>
<SCRIPT LANGUAGE="JavaScript">
var rand= Math.round(Math.random()*5);
namemc = "my_menu"+rand+".swf"
//alert(rand);
</SCRIPT>
<META http-equiv="Content-Type" content="text/html; charset=windows-1251">
</HEAD>
<BODY bgcolor="#FFFFFF" text="#000000">
<object classid="clsid:d27cdb6e-ae6d-11cf-96b8-444553540000" codebase="http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=6,0,0,0" width="160" height="250" id="my_menu9" align="middle">
<param name="allowScriptAccess" value="sameDomain" />
<param name="movie" value="namemc" />
<param name="menu" value="false" />
<param name="quality" value="best" />
<param name="bgcolor" value="#ffffff" />
<embed src="namemc" menu="false" quality="best" bgcolor="#ffffff" width="160" height="250" name="my_menu9" align="middle" allowScriptAccess="sameDomain" type="application/x-shockwave-flash" pluginspage="http://www.macromedia.com/go/getflashplayer" />
</object>
</BODY>
</HTML>