@walshinik, this is pretty anecdotal, and it happened to me on a Windows laptop using Firefox with flash enabled, but I was on a reputable "training video" site (you'd recognize the name), and i was able to get something that looked like an antivirus checker. The thing took over my system in record time, and not even pressing the power button in a panic could get rid of its extensive presence in Windows (task manager, login screen, etc.).
I was a pretty embarassing trip to IT and the guy had a grin on his face all the while he was checking out my problem. Of course I told him I had no idea what had happened, and anyway there was no reason for a girl to be watching "training videos", right?
So, um.. be careful!