Get a popup blocker, shouldnt be a problem since all browsers now days have one, if your on IE get service pack 2 or swtich to firefox. or install google toolbar.

Use Firefox with built in pop up blocking

The problem is not in any thing that you are doing. The Sears ad is hijacking you and pulling you to their site. I had the same problem and emailed a complaint to Yahoo. Seems that Yahoo would not want their advertiser taking away customers without their consent. Unfortunately, the pop-up blockers don’t work in this case. The best thing is to boycott advertisers that use those tactics. I intend to bypass Sears for the holiday shopping this year.

Go to
I use that program.It works very well.
Block pop-up ads and remove spyware .

Try
Prevents from any unwanted popup ads immediately and automatically.
Block 100% unsolicited popups.
Run quietly in your system tray using very little resources.