This code show how to create snowfall in flash cs4

u can also use the code for cs3

package {

	import flash.display.MovieClip;
	import flash.events.Event;
	import flash.filters.BlurFilter;
	import druva.NumberUtil;
	import flash.utils.Timer;
	import flash.events.*;

	public class snowfall extends MovieClip {

		private var t:Timer;

		public function snowfall():void {
			t=new Timer(80);
			t.addEventListener(TimerEvent.TIMER, createParticle);
			t.start();
		}

		private function createParticle(e:TimerEvent) {

			var mc_particle:MovieClip =new MovieClip();
			mc_particle.graphics.beginFill(0xFFFFFF,1);
			mc_particle.graphics.drawCircle(0, 0, NumberUtil.getRand(3,5));
			mc_particle.graphics.endFill();

			mc_particle.x=NumberUtil.getRand(-60,600);
			mc_particle.wind = (NumberUtil.getRand(0,1) == 1) ?  'left' : 'right';
			mc_particle.filters=[new BlurFilter(10,10,2)];
			mc_particle.addEventListener(Event.ENTER_FRAME, moveParticle);
			addChild(mc_particle);

		}

		function moveParticle(e:Event):void {
			var targ:MovieClip=MovieClip(e.target);

			if (targ.y>400) {
				targ.removeEventListener(Event.ENTER_FRAME,moveParticle);
				removeChild(targ);
				return;
			}

			targ.y += 1;

			if (targ.wind=='left') {
				targ.x += 0.4;
			} else if (targ.wind== 'right') {
				targ.x -= 0.4;

			}

		}

	}
}