I have an unwanted pop-up that keeps appearing on my desktop, usually some sort of music ad. When I click on the "X" in the upper left hand corner to close the pop-up, it will open Internet Explorer going to a webpage onclickads.net. I have always immediately closed Internet Explorer (I routinely use Firefox or Chrome for my browser, virtually never I.E.) and not let it finish loading the page. Likewise, I have never clicked on anything except the "X" on the pop-up. It isn't real intrusive, it appears once or twice a day, and when I close the I.E. window nothing else happens until the next time the pop-up appears.
I have Windows Vista Ultimate SP2 32 bit OS. My primary anti-virus is McAfee All Access - Total Protection, the Anti-Virus/Anti-Spyware portion of that is V18.0, Build 18.0.177, last updated today 8/12/2015. I ran McAfee scan in safe mode w/networking and then Stinger32; both found no problems.
I also have Malwarebytes Anti-Malware V18.104.22.1687, build date 6/18/2015 10:38:55 AM also updated today and it likewise finds no problems.
How do I go about getting rid of these pop-ups?