You've picked up a particularly nasty piece of malware called a browser hijack. Specifically this piece of software lurks in the background watching everyplace you go on the web and when you try to access a particular type of website (such as a search engine) that its' customers are competing against, it will redirect you to its' customer's website instead.
Download a free piece of software called
Spybot Search & Destroy, along with any updates for it. Install it and any updates you've downloaded, run and it should get rid of most of the problems on the first pass. Be sure to use its' Immunize feature to help lock down Internet Explorer.
Also, I highly encourage you to switch to
Mozilla Firefox for web browsing. In fact, you may have to as some of these browser hijacks will prevent you from getting to the Spybot S&D homepage.