Its an ad popup thats snuck into the rotation. Its not a virus or anything to worry about. Just a nuisance. Were trying to track it down.
If you're still seeing it now, try clearing out your temp files.
I did a few minutes ago, working better now.
Google is tricky, you have to go to Chrome and clear that history, then follow a link at the bottom of a page to a different record of your searches. For anyone else who has to deal with it, you have to clear both.