1) Please change the password of your internet connection immediately;
2) Install & run a good spyware remover, e.g. Ad Aware (You can get it at Download.com or google it from the Lavasoft site);
3) Open your browser, if it's Internet Explorer, go to 'Tools'>Internet Options and then click the 'Use Blank' tab under 'Home Page'.

See if it helps. Best of luck !