Virus not detected by scan

Hello,

I just downloaded the free software because I have had a virus for a month or so. I was previously using Mackeeper, but learned/realized that it isn’t that great. Regardless, I just completed a full system scan that with 0 infections reported. Problem is I am still experiencing the same issues. I frequently have video pop ups (advertising, lower right hand corner of screen, approximately 2" x 2" in size) as well as page pop ups. Does any one have any suggestions how to remove these? Is it possible that Avast! can help. I apologize in advance for my limited ability to explain/describe my issues, for I’m not even sure that this is the correct place to post!

I’d appreciate any advice. Thanks!