This is a general suggestion and it helped many people. If nothing works, you can either reinstall Safari or just use another browser like Firefox, Camino, Flock, etc.
I am running OS X 10.5.8 with Safari 4.0.3 on an IMAC. Processor is 2.4 GHz Intel Core 2 Duo. I use the private browsing mode with pop-up blocking activated and cookies denied. Still I get unsolicited business advertisement windows and phone chat windows every time I click a link on whatever page I'm viewing. I have run DNSChanger remover, MacScan, iAtivirus and ClamXav and found nothing. Appreciate any help you can give me.