So finally I've solved it. Here are some steps, in case someone need.
- Using Spy++ I've pointer to the window and found related process. In my case it was FFPRODUCTUPDATER. [Spy++ is part of Visual Studio, but you can find alternatives, just google it]
- Searched hard drive for the process. In my case it was some MP3 cutter/jointer, which I had downloaded few days ago. I uninstalled the program, restarted and the problem of course disappeared.
I suspect the program was either malware or adware/spyware.