sounds like you got your self a hijaker...I would start with downloading
ccleaner
http://www.filehippo.com/download_ccleaner
then
advanced system care http://www.iobit.com/advancedsystemcareper.html
then
Spybot search and destroy
http://www.safer-networking.org/en/spybotsd/index.html
See if that helps if not sounds like it may have embedded itself in your registry. If that's the case download hijack this and do a a scan and save log file DO NOT REMOVE ANYTHING and post log file here and I will take a look at it and see what you have that should not be there.
http://free.antivirus.com/hijackthis/
You get more with a kind word and a two by four, than you do with just a kind word. ~ Gaffer