I was trying to open a video online today, when a number of overlaid screens appeared telling me the computer had been locked and I had a trojan. It gave a screen purporting to be Microsoft asked for access. Against the instruction, I turned the computer off and left it off for an hour or so. When I powered up, before opening the browser, I ran a deep scan with my antivirus. It ran for an hour and a half, but found 17 suspect files which it then deleted. Haven't had any problems since I restarted.