You may have a virus, is your favorite URL file the same?
Or just some nasty spyware. Here’s 3 free progarams I use. The last two even repaired some serious damage that wasn’t a virus but just as bad;
http://www.download.com/Ad-Aware-2008/3000-8022_4-10045910.html
http://www.superantispyware.com/
http://www.malwarebytes.org/mbam.php