Код:
<script language="JavaScript1.2">   
grphcs=new Array(2)   
Image0=new Image();   
Image0.src=grphcs[0]="http://fantasyflash.ru/script/web/image/sneg1.gif";   
Image1=new Image();   
Image1.src=grphcs[1]="http://fantasyflash.ru/script/web/image/sneg1.gif";   
Amount=25;   
Ypos=new Array();   
Xpos=new Array();   
Speed=new Array();   
Step=new Array();   
Cstep=new Array();   
ns=(document.layers)?1:0;   
ns6=(document.getElementById&&!document.all)?1:0;   
if (ns){   
for (i = 0; i < Amount; i++){   
var P=Math.floor(Math.random()*grphcs.length);   
rndPic=grphcs[P];   
document.write("<LAYER NAME='sn"+i+"' LEFT=0 TOP=0><img src="+rndPic+"></LAYER>");   
}   
}   
else{   
document.write('<div style="position:absolute;top:0px;left:0px"><div style="position:relative">');   
for (i = 0; i < Amount; i++){   
var P=Math.floor(Math.random()*grphcs.length);   
rndPic=grphcs[P];//osw   
document.write('<img id="si'+i+'" src="'+rndPic+'" style="position:absolute;top:0px;left:0px">');   
}   
document.write('</div></div>');   
}   
WinHeight=(ns||ns6)?window.innerHeight:window.document.body.clientHeight;   
WinWidth=(ns||ns6)?window.innerWidth-70:window.document.body.clientWidth;//fantasyflash.ru   
for (i=0; i < Amount; i++){   
Ypos[i] = Math.round(Math.random()*WinHeight);   
Xpos[i] = Math.round(Math.random()*WinWidth);   
Speed[i]= Math.random()*5+3;   
Cstep[i]=0;   
Step[i]=Math.random()*0.1+0.05;   
}   
function fall(){   
var WinHeight=(ns||ns6)?window.innerHeight:window.document.body.clientHeight;   
var WinWidth=(ns||ns6)?window.innerWidth-70:window.document.body.clientWidth;   
var hscrll=(ns||ns6)?window.pageYOffset:document.body.scrollTop;   
var wscrll=(ns||ns6)?window.pageXOffset:document.body.scrollLeft;   
for (i=0; i < Amount; i++){   
sy = Speed[i]*Math.sin(90*Math.PI/180);   
sx = Speed[i]*Math.cos(Cstep[i]);   
Ypos[i]+=sy;   
Xpos[i]+=sx;   
if (Ypos[i] > WinHeight){   
Ypos[i]=-60;   
Xpos[i]=Math.round(Math.random()*WinWidth);   
Speed[i]=Math.random()*5+3;   
}   
if (ns){   
document.layers['sn'+i].left=Xpos[i];   
document.layers['sn'+i].top=Ypos[i]+hscrll;   
}   
else if (ns6){   
document.getElementById("si"+i).style.left=Math.min(WinWidth,Xpos[i]);   
document.getElementById("si"+i).style.top=Ypos[i]+hscrll;   
}   
else{   
eval("document.all.si"+i).style.left=Xpos[i];   
eval("document.all.si"+i).style.top=Ypos[i]+hscrll;   
}   
Cstep[i]+=Step[i];   
}   
setTimeout('fall()',20);   
}   
window.onload=fall   
//-->   
</script>

Вставлять нужно в самый конец после </BODY></HTML>