Upload the contents in the zip file attached to your root folder.
Then in your admincp :
Styles -> Modify -> Fonts/colors/etc -> Head Insert :
Add
PHP Code:
<script language="javascript" type="text/javascript" src="snowflake.js"></script>
Styles -> Modify -> Fonts/colors/etc -> Body Tag :
Add
PHP Code:
onLoad="fall()"
somewhere within the body tag.
Should give you the effect you want.
There are some settings in the .js file that you can amend for number of seconds you want it to run for, as well as how smooth it runs.
Isn't done by me, just something I have had on my hdd for the longest time and cannot remember where i got it from.
Hope it helps.