Bummer....Finding the culprit will require the use of tools not allowed in this forum. Along with the expertise to use the tools.
Start a new topic after creating a DDS log by following instruction #6 found here: Preparation Guide For Use Before Using Malware Removal Tools and Requesting Help - Virus, Trojan, Spyware, and Malware Removal Logs
Post the DDS log along with a description of the problem in the Virus, Trojan, Spyware, and Malware Removal Logs Forum - BleepingComputer.com
Do not bump your topic once it is posted. Wait for a response. It could be a few days.
EDIT: Try one other thing....open CCleaner and click on Startups. At the top of that page you will see a button titled Tasks. Click on that and
post the lists of tasks by clicking on the button at the bottom of the same page with the task list open. Clicking on the button at the bottom
of the page will allow you to copy and paste the list of Tasks back here. Please do that.
You can safely disable any of those tasks especially one that you think might be causing the ad to appear. Or wait until I review them.