I got it several times last year through some LJ page but I managed to close the browser in time.
There was one that was really bad. Sorry I don't remember the name. My brother got it somehow. One day it just showed up on the screen and took over everything. We couldn't do anything, couldn't browse, couldn't close down the browser, no taskbar, mouse did no longer work.All we could do was like letting that darn program scanned the computer and we must buy its program.So we took the extream measure to force close down the computer. But when we started the computer again it took only about 30 second or something to come back and took control over the computer again. I checked for the solution on Google and follow the some advice from some kind soul to take it away. I think he got it through some Asian chat forum or Game forum.
I did try some cyber defender program but when I saw that it counted some Window program as malware so I stopped. I use free version of Avast and so far I have no problem with anything. Norton is great too but I find them make the computer rather slow but I could be wrong though.