Guys DNS Unlocker is most likely showing ads in your browser because of hijacked/poisoned DNS, no need to reinstall the OS. Check your DNS servers on your OS and router to check they are what you expect them to be (either automatic or values provided by a dns provider like Google DNS or OpenDNS).
In my case I found that the most likely cause is a poisoned upstream DNS server providing incorrect values for www.google-analytics.com, my DNS settings were intact. You can install Malware Bytes and activate the Premium features, and/or install an adblocker and block *.google-analytics.com. Blocking *.dnsqa.me will also help.