I haven't had one particularly nasty virus, but rather a collaborative clusterfuck of at least 20 viruses in one case, and 380+ (All the same virus though) on another.
First one happened when i tried downloading a keygen for a game. When i opened it, my PC got buttfucked. It never fully recovered before i formatted it.
As for the second one, i don't actually know how it got on my PC, but what i do know is that it infected nearly every EXE file on my PC. This didn't have any notable effects, oddly, except for AVG constantly bawwing about the viruses, which was rather disruptive in and of itself. When i got around to AVG'ing the viruses, all the infected EXE files got broken, leaving me with a shitton of useless games on my PC. Had to format it once again.