I am a happy man today !
I have been huanted by the same problem for the past 3 weeks. I am not a computer guy. I tried Norton, Ad-aware, etc. with no result. I've tried Yahoo Toolbar recently with no result. However I did an update today, and then scanned my computer, and found a whole bunch of things. I removed all of them, restarted the computer 2 times, and left it on for the last 2 hours - NO POP UP.
Toolbar is free for download from Yahoo - since it is a Yahoo product, I am extreme comfortable with it.
Good luck.

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Cruiser.