Any chance you could clarify this a bit further? I don't really understand what you're trying to achieve here!
Do you mean as soon as the swf has loaded, you want it to take a screenshot of the entire web-page??
If so, it sounds a bit unlikely to me as the swf will not be able to see anything external to itself. So it wouldn't be possible to take a screenshot of the entire web-page directly from Flash/AS3 (at least as far as I'm aware!)
You MAY be able to connect your swf to an external server-side script (javascript/php) which could retrieve the HTML for the page and generate an image of it. But again, I'm really not sure!
If it is possible, it would probably require quite a lot of work and resources to set up. What do you intend to do with the screenshot after you've got it?