When you explore the internet you pick up all sorts of adverts and such without noticing, ofter you get things called adware which spy on you and then send reports about your internet usage to advertisers.
Adware is usualy harmless but it does slow down your internet and computer a lot.
I advice you to try AD-aware from http://www.adaware.com to search for and remove the advertising rubbish and spyware/adware you have picked up.
