1.Clean your browser cache,history,cookies etc. and set browser to clean that data automaticaly uppon exit if it can ( do not know what browser you use )
2.Unchek unvanted startup programs, I use CCleaner for that and it is very good program for overall cleaning of PC, so use it for that purpose too!
2.Install,scann and remove adavare using this awesome tool: AdwCleaner from this site: " http://www.bleepingcomputer.com/download/adwcleaner/ "
3.Clean all your "Restore point" data bcz if you do in the future "Sysem Restore",it is possible for maleware to return.
4.Scann with free Malwarebytes Antimalware and Hitman Pro scanners just to be shure that you PC is clean
5.Install this browser extensions: a) BitDefender-Traffic Light b) Adguard and check (in the settings) appropriate options
6.Check in you browser settings, that in "startup page" or "home page" is what you want, myne is " https://google.hr/ "
7.Instal good AV program ( just one of this ) like few of this one: Panda,Avira,Avast,Comodo,BitDefender...
8.You can also reset hosts file or mabye is beter to replace it with modified version who blocks hundreds of malcious sites, check this site and download modified hosts file in addition to your version of windows ( http://winhelp2002.mvps.org/hosts.htm )
GOOD LUCK ;)