also try computer security section,.?link=list&sid=396546062

you may have spyware, which may be hard to get rid of if bad,
suggest download anti-spyware tools and anti-virus and firewall, if you have those already, firewall should be on while the web, and antivirus,

first empty browser cookies and temporary internet cache files, and as well temp folders,

start by doing spyware scan,

if anything found and can not be removed cause of being in use, may need to restart computer in safe mode, (F8), then scan again,

if nothing found then do anti virus scan,

and then once problem is resolved, maintain firewall and antivirus programs and leave running in background, and always clean up browser history and cache more often,

if also need there are browser internet options in control panel that can be set to block sites, and increase security and content, as well filters to add-on ,
..