Così funziona perfettamente!!!

codice:
<%
Response.CacheControl="no-cache"
Response.AddHeader "Pragma","no-cache"
Response.Expires=0
randomize
rand_n=int(rnd*9999)
%>

.......

<OBJECT classid="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000"' codebase="[...]omissis[...]"' WIDTH=640 HEIGHT=486>'
<PARAM NAME=movie VALUE="filmato.swf?<%=rand_n%>">
<PARAM NAME=loop VALUE=false>
<PARAM NAME=bgcolor VALUE=#FFFFFF>
<PARAM NAME=menu VALUE=false>
<PARAM NAME=quality VALUE=high>
<EMBED src="filmato.swf?<%=rand_n%>" loop=false menu=false quality=high bgcolor=#FFFFFF WIDTH=640 HEIGHT=486 
TYPE="application/x-shockwave-flash" 
PLUGINSPAGE="http://www.macromedia.com/go/getflashplayer">
</EMBED>
</OBJECT>