Sounds like you have gotten something nasty.
http://www.windowsbbs.com/forumdisplay.php?f=41
Go to that link...read the instructions and welcome thread.
Find the hijack this program and download and then post the findings on that board.
It is probably a program hid on your computer that is hard to find and it just starts doing popups if you are connected to the net (like with a cable ISP you may not be surfing the net but there is always a connection as long as the power is on and the modem is connected.)
Hopefully they will be able to find the problem and help you, I know they have helped me once.