Просто вставить в head допустим.

                        
<script>

var snowmax=35

var snowcolor=new Array("#aaaacc","#ddddFF","#ccccDD")

var snowtype=new Array("Arial Black","Arial Narrow","Times","Comic Sans MS")

var snowletter="*"

var sinkspeed=0.6

var snowmaxsize=22

var snowminsize=8

var snowingzone=3

var snow=new Array()

var marginbottom

var marginright

var timer

var i_snow=0

var x_mv=new Array();

var crds=new Array();

var lftrght=new Array();

var browserinfos=navigator.userAgent 

var ie5=document.all&&document.getElementById&&!browserinfos.match(/Opera/)

var ns6=document.getElementById&&!document.all

var opera=browserinfos.match(/Opera/)  

var browserok=ie5||ns6||opera



function randommaker(range) {		

	rand=Math.floor(range*Math.random())

    return rand

}



function initsnow() {

	if (ie5 || opera) {

		marginbottom = document.body.clientHeight

		marginright = document.body.clientWidth

	}

	else if (ns6) {

		marginbottom = window.innerHeight

		marginright = window.innerWidth

	}

	var snowsizerange=snowmaxsize-snowminsize

	for (i=0;i<=snowmax;i++) {

		crds[i] = 0;                      

    	lftrght[i] = Math.random()*15;         

    	x_mv[i] = 0.03 + Math.random()/10;

		snow[i]=document.getElementById("s"+i)

		snow[i].style.fontFamily=snowtype[randommaker(snowtype.length)]

		snow[i].size=randommaker(snowsizerange)+snowminsize

		snow[i].style.fontSize=snow[i].size

		snow[i].style.color=snowcolor[randommaker(snowcolor.length)]

		snow[i].sink=sinkspeed*snow[i].size/5

		if (snowingzone==1) {snow[i].posx=randommaker(marginright-snow[i].size)}

		if (snowingzone==2) {snow[i].posx=randommaker(marginright/2-snow[i].size)}

		if (snowingzone==3) {snow[i].posx=randommaker(marginright/2-snow[i].size)+marginright/4}

		if (snowingzone==4) {snow[i].posx=randommaker(marginright/2-snow[i].size)+marginright/2}

		snow[i].posy=randommaker(2*marginbottom-marginbottom-2*snow[i].size)

		snow[i].style.left=snow[i].posx

		snow[i].style.top=snow[i].posy

	}

	movesnow()

}



function movesnow() {

	for (i=0;i<=snowmax;i++) {

		crds[i] += x_mv[i];

		snow[i].posy+=snow[i].sink

		snow[i].style.left=snow[i].posx+lftrght[i]*Math.sin(crds[i]);

		snow[i].style.top=snow[i].posy

		

		if (snow[i].posy>=marginbottom-2*snow[i].size || parseInt(snow[i].style.left)>(marginright-3*lftrght[i])){

			if (snowingzone==1) {snow[i].posx=randommaker(marginright-snow[i].size)}

			if (snowingzone==2) {snow[i].posx=randommaker(marginright/2-snow[i].size)}

			if (snowingzone==3) {snow[i].posx=randommaker(marginright/2-snow[i].size)+marginright/4}

			if (snowingzone==4) {snow[i].posx=randommaker(marginright/2-snow[i].size)+marginright/2}

			snow[i].posy=0

		}

	}

	var timer=setTimeout("movesnow()",50)

}



for (i=0;i<=snowmax;i++) {

	document.write("<span id='s"+i+"' style='position:absolute;top:-"+snowmaxsize+"'>"+snowletter+"</span>")

}

if (browserok) {

	window.onload=initsnow

}

</script>