Windows 2000 broblem

I manage an cafe that uses broadband connection i don’t know which type but it is radoio link as we call it.The server the shares the connection has windows 2000 professional installed on it and any time i try browsing the internet the internet explorer is automatically set to mail.yahoo.com but when i luch the internet explorer this web www.iwantsearch.com page keeps appearing and also changes the home page from mail.yahoo.com to www.iwantsearch.com.What could be the problem wshat remedy should i deploy to solve the problem.