Because the Background is the best part of the Computer.
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[SIZE="5"]Shall we?[/SIZE]
Ok, start by making an animation or some actionscripted thing, what you wish.
I made a Clock.
See?
Now at the end of you work done, test it (Ctrl+Enter) to see if works fine.
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[SIZE="5"]How to?[/SIZE]
It's easy!
After your swf. file selection is done, just go to Publish settings, in the File tab;

Ok, now pay attention. Some of you get lost in this part.
After clicking it will appear to you a settings box;

This one, in a standart definition the two first ones should be selected. Let them be. Those will create two separate files. One will contain your Movieclip, and the other the information you need to mantain your background active.
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[SIZE="5"]The next step[/SIZE]
Ok, you now have those 3 upper tabs

Obviusly, the Flash tab for Flash settings and the HTML tab for the background settings.
Well, let's leave the Flash for now. It's the HTML one we want.

Let the template the way it is, the dimensions too, put your configurations like the Playback ones. Select you quality, but when it's done you can change it anyway (Right click+Quality), Choose the ''Alpha'' level you want in your .swf file. HTML Alignment, choose where in the background you want to have your .swf file, I've got mine in the Upper right corner.(Just choose Right for that, if you want.)
Choose your scale, and change the Flash Alignment for what you want.
The ''Show warning messages'' box, is for warning whe you have an error in you actionscript.

If you tested your movie before and corrected all your errors, it won't happen.
At the end of all your settings done, click Publish;

Not Ok;

Got it?
Sure you did, >:3
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[SIZE="5"]After the storm[/SIZE]
Now comes the easy part.
If you go to your desktop you should see this two little icons.

It's a good sign!
Next you'll just have to open your computer settings and go to Desktop;

And choose the non-Flash Icon, the one I have selected.
Click OK.
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[SIZE="5"]Congratulations![/SIZE]
You've done it. You now have the best desktop of them all.

[SIZE="1"]P.S; Slow computers may notice some, very lag. And sorry for anything that's not quite well explained.[/SIZE]