If when you start your browser it loads a different web site instead of your normal homepage, it is possible a malicious program has hijacked your browser and set a different homepage.
To change it back, click on the Tools menu then Internet Options. On the General tab, under the section Home page you should see an Address box. Type in the address of the site you want (or click Use Current for the site you are currently viewing) and click OK.
If you find your homepage keeps changing on its own, you will need check for spyware.