foxfire42,
I have had this happen to me several times in the last few months, the last time was a couple of weeks ago.
This last time, it started as soon as I install the free version of Malwarebytes and AVG Free, I am not saying for certain that it was related, but I believe that it was. I have since disabled both.
What I did was I googled removal or uninstall whatever the adware name was that seemed associated with the ads. I usually use googlechrome, I wound up using Revo Uninstaller Pro but also on the right side of the top of the screen there is a Customize and control Google Chrome button next to the tool bar and had to reset some defaults.
I can't recall what adware that I was chasing, but if you post the name of what you are seeing and what browser you are using I can do some research.
Darryl in Fairbanks