And it's probably the #2 reason why you shouldn't be using Internet Explorer (#1 being security issues).
Poke around a bit, and you'll find Bob's list of spyware removal tools. Which takes care of cleaning up the mess you've already got. The next step is to prevent the mess from ever happening again, and to do that you need to use an alternative browser such as Mozilla/Firefox or Opera. You keep IE around for those very few rare sites that won't work with anything else, and Windows Update. Everything else, you use one of the other browsers for. A bigger step in the same direction would be to get rid of any email client that has "Outlook" in the name, for the same reasons as not using IE, only you NEVER use that again.
Should you fail to heed this advice, you are putting yourself at greatly elevated risk of programs hijacking your browser again, and that's the best case scenario. Worst case scenario, is someone exploiting a security hole in IE, using your computer to do something illegal, and leaving you holding the bag. Most likely scenario is that spyware will continue to get onto your system and you'll have to keep removing it constantly. And not all spyware is as obvious as search bars. Some just sit silently, watching what you do, and sending back data about your habits to sleazy companies. Some will even steal credit card numbers, bank account numbers, and passwords by capturing your keystrokes. If you want to take that risk, keep using IE. Otherwise, download the three alternatives I mentioned, try them each out for a few days, and use the one you like best.