On the Internet Explorer Page, go to the Tools menu, and select 'internet options'. In here you can change the home page (the page that you automatically go to when opening up a browser). This might be the reason but no guarantees.
Sounds like a malware virus. I have spent all day yesterday trying to get rid of one from my computer. I run Mcafee virus protection and firewall and it still got past. Mcafee tried unsucccessfully to help me get rid of it yesterday and we are trying again today. Here is a link to a website with step by step instruction on how to get rid of it (if it is the same one).