Annoying Ad.

[Bitmap Frogs]

Avast is shit. It's not a free antivirus, it's a free poor quality antivirus. And it's specially weak against these kind of adwarez; just a couple weeks ago I removed a similar malware from my room mate, who by the way was also using Avast at the time.

Here's what you should do: open the task manager and go through each of your process with a process database (there are some online) to identify the culprit. Once you do, use microsoft's tool for process managing to track the archive that spawned the process and just trash it.

edit with relevant links:

http://www.processlibrary.com/

http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx

edit with suggestion:

migrate to linux or mac