Use task manager to check that your machine's not running anything that looks like: Bargain Buddy, Bullseye Networks or Windows AdControl. If it is then there's a good chance that those banners are actually iFrames overlaying the real ones. It's a new adware/scumware scam exploiting (surprise, surprise) security holes in IE6.

If you do have any of that stuff there's plenty of advice out there on how to get rid of it, but be warned I spent 12 hours cleaning 3 machines at work! I tried to persuade my boss to send them a bill for my time - doing some whois shows they're all legal entities in the US - maybe if we all got together and sued them for our wasted time they'd think twice about their tactics!