Because it's not a virus. It's "spy ware" or "ad-ware", stuff that either records what you do on your computer or tries to sell you something. Like Peter said, it's called "contextual advertising", it's making links of words it recognizes and tries to get you to click them. SpyBot and AdAware should do the trick. And even if I didn't say it, it's probably gotten on your machine because you're using IE. It's Active X-controls let in all kinds of spy- adn adware programs without asking you anything.

Ice