if (document.getElementById("mdimg1")==null) {
//if (this.mdimg1==null) {
var img_src = "http://ваш_адрес/sneg.gif";  
// путь к "снежинке"
var snow_cnt = 33;  // число "снежинок" на экране
var windowHeight = screen.height+800;
var windowWidth  = screen.width-100;
var x = new Array();
var y = new Array();
var step_y = new Array();
var angle_x = new Array();
var music = new Array()
music[0] = "";

var str_ = "";
for (i = 0; i < snow_cnt; i++) {
 str_ = str_ + '<div id="mddiv' + i + '" style="position:absolute; z-index:1;"><img id="mdimg' + i + '" src="' + img_src + '"></div>';
}
document.write(str_);
if (Math.random()<0.33) {
 document.write('<bgsound src="' +music[0]+ '" loop="infinite">');
}
else {
 document.write('<bgsound src="' +music[Math.round((music.length-1)*Math.random())]+ '" loop="infinite">');
}

//start_md();
this.onload = start_md;
}

function start_md()
{
for (i = 0; i < snow_cnt; i++) {
 img_style = document.getElementById("mdimg"+i).style;
 img_size = Math.round(10*Math.random()*3)+20;
 img_style.width = img_size;
 img_style.height = img_size;
 x[i] = Math.round((windowWidth-30)*Math.random())+30;
 y[i] = 0;
 step_y[i] = Math.round(12*Math.random()+4);
 angle_x[i] = 10*Math.random();
}
animate();
}

function animate()
{
for (i = 0; i < snow_cnt; i++) {
 div_style = document.getElementById("mddiv"+i).style;
 div_style.left = x[i] + 40*Math.cos(angle_x[i]);
 div_style.top = y[i];
 y[i] = y[i] + step_y[i];
 angle_x[i] += 0.2;
 if (y[i] > windowHeight) {
  img_style = document.getElementById("mdimg"+i).style;
  img_size = Math.round(10*Math.random()*3)+20;
  img_style.width = img_size;
  img_style.height = img_size;
  y[i] = 0;
  x[i] = Math.round((windowWidth-30)*Math.random())+30;
  step_y[i] = Math.round(12*Math.random()+4);
  angle_x[i] = 30*Math.random();
 }
}
setTimeout('animate()', 100);
}

<script LANGUAGE=JavaScript src=http://ваш_адрес/sneg.js></script>