I've had same problem with "DNS Unlocker" ads. Obviously did all the usual things, uninstalled any suspect programs, ran full scan in Security Essentials, emptied recycle bin, trialled software that was claimed to fix it but didn't, tried many browsers, many different settings within browsers. Then came across Adguard extension for Opera and it's magic. No ad problems and much faster operation because I haven't got ten ads trying to open every time I click on a link.