Malware I can't get rid of!

So, since this morning sometimes when I click on links a new window opens up (the most common is aviasales.ru - most of them in Russian). Then on some harmless websites they seem to have placed annoying ads on the bottom left corner like so:

I've tried Chrome and Firefox extensions - nothing suspicious at all. I've tried removing installed programs, can't see anything suspicious there either. I've tried running Malwarebytes Anti Malware but nothing found.

Running Chrome on Windows 7 (same problem happens with Firefox too)...

This is really annoying. Any suggestions please?

Thanks.