I'm sorry. I'll behave and put the right ones back. Just Kidding.
This sounds like spyware from a free downloaded game or screensaver. Some spyware is designed to replace popups and banners with ones of it's choosing.
Another possibility is a cable modem or DSL without a firewall. A port-sniffer might have found an open port and they got on your computer without you even knowing it.
Pop Up and Spyware