Intrusive Advertisements

A few days ago I started getting intrusive pop-up ads while browsing the internet.  New internet explorer browser windows open by themselves and display advertisements and sometimes malicious sites.  The Norton toolbar, which I have enabled, is disabled in these strange advertisement windows.  I ran a full scan with Norton and a scan with Malwarebytes but nothing was detected.  I did not install any new programs recently, and this problem just started happening a few days ago.  I am using a windows 7 pc, and have all of the latest windows updates installed.  I would be grateful for any suggestions on how to solve this problem.