Help Stopping popups!!

A friend of mine goes online and every 15 minutes or so a pop up appears saying he has a trojan /spyware virus on his pc. I installed the Lavasoft Adaware program and Spybot Search and Destroy and a popup blocker program. They both found many spyware programs and I deleted them. The problem is he is still having these trojan/spyware popups appearing every once in awhile and when you click on it it goes there and it's a "purchase this program" website. How can we stop these from appearing? Also, sometimes a dialog box appears showing nude women.I thought this was some kind of popup but it still appears sometimes. What program can he download to prevent those from appearing?
Thanks for any help,