This is most likely a browser hijacker (think virus) Download and install Hijack This
http://free.antivirus.com/hijackthis/ and say do a system scan and save a log file. Copy the contents of the log file and paste it here and we can see if there is something awry. DO NOT click fix this, or fix anything or check off any boxes until we look through the list.
Hijack this is an extremely powerful program, but isn't very user friendly as far as telling automatically what is good and bad. It looks for anything that interacts with your browsers, so some is stuff you want and some is stuff that you don't want
EDIT: Also, get an anti virus, but don't pay for one. There are plenty of free antivirus programs and firewalls that perform as well or, in many cases, much better than anything offered by Semantic or McAfee