How to put ads on every web page?

I am wondering how to put ads like the software Hotspot Shield does. It puts ads on the top of every web pag开发者_开发技巧e. I don't want in a specific language like vb or c++ i just want how in any language.

Examples:

• http://ishtechnology.com/wp-content/uploads/2010/10/Blog-Ad-added-by-Hotspot-Shield-e1288306756847.png
• http://www.raymond.cc/images/hotspot-shield-advertisements.png

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Beside from the obvious evilness of this, the solution is quite simple : install a transparent http proxy on the machine and decorate each and every page with the html code required to display add.

Did I say that kind of behaviour was simply bad ? It indeed defaces web pages, for the sole purpose of displaying adds that will rarely be clicked.