Fraudware: IE Defender
I see on one of my mailing lists that there has been an upsurge in reported infections by a fraudware known as IE Defender.
A description of the fraudware (including screenshots) and cleanup instructions can be found here - thankfully it's easy enough to remove.
Note the various tricks the product uses to fool victims into purchasing the product - the pop-ups warning of a non-existent infection on computers, the fake info-bar and web browser error page, and the hijacked Google and Yahoo searches with fake results.
While you're at bleepingcomputer, you should check out the other information that is available there - it's an excellent resource of descriptions and screenshots of various malware and fraudware, as well as clean-up tools.
Thanks to Lawrence for putting the article together...