Unsolicited sites when online

When I log onto the web, afetr a few minutes, I get unsolicited adevrtidsing screens opening up, including Yell, Traffic MarketPlace, and various others. Sometimes there is obviously a window there, but it cannot be accessed and only deleted by accessing the background programmes running via Ctr-All-Del. Directories created in my History file are “xads.offeroptimizer(xads.offeroptimizer.com)” and “xlime.offeroptimizer(xlime.offeroptimiser.com)”. There doesn’t appear to be anything in startup initialising these sites.

How can I stop them opening up ?