If they appear on every page you go to where you would not normally expect them, such as here or eBay or the BBC, then they'd most likely be caused by adware. Do all the usual anti-adware things to get rid of the malicious software. If you don't know what those things are, post back here and we'll help you.
If they appear on specific sites, then you'll have to live with it, it's just advertising and you can't turn it off. You may be able to find out the address of the server that's serving up the ads and block that either by a browser extension like Adblock Plus if you use Firefox or by adding an entry to your hosts file which will work on any browser.