Oh, the "code for it" is all you want? Let me snap my fingers <snap!>
Well, since magic fails... you have to realize that an ad system is quite a bit more complex than adding a few "simple codes" to your pages. I would suggest that you apply to a contextual advertising system, such as ContextWeb or (though typing the very word makes me want to puke) IntelliTXT.
I also suggest that implementing "hover" ads on your site is a dead giveaway to your users that your site content is just fluff, meant to bait users, that the real reason for the site is to carry ads. In other words, if you must have advertising, use something less blatantly evil than "hover" ads (otherwise known as in-line contextual ads).
Basically, the ad content is placed in a DIV, and when the page loads (body's onload event), they reposition the DIV.