try and remove your "temporary internet files & cookies and run yourgoogoo said:
spyware program and remove any spyware that you might have on your system.
you could have a virus so do a online virus scan make sure you do not
have any other programs running while the scan is in progress.
here are two sites you can use for the scan and also a spyware program.
http://housecall.antivirus.com/housecall/start_corp.asp
http://www.download.com/3000-2094-10045910.html?legacy=cnet