There is a Trojan Horse appearing on some websites that claims that it "needs an updated driver" to display a video or similar file. It won't let you back out from installing the "upgrade," opening the same window again and again. But it is no upgrade; it is a Trojan Horse designed to take over your computer.
I encountered several sites proffering Woodstock footage that set the trap, but there are others too. Once you enter the page, the trap is already sprung. The install window opens immediately.
Once you see the install window that won't let you refuse installation, the ONLY way to keep it from installing the malware is to Ctrl-Alt-Del and close your browser through the task manager. You will have to tell it you want to close it anyway when it says the browser is waiting for your input.
If you accidentally installed it, the Symantec antivirus program can remove it.