So you are signed up on Facebook.com and you get an e-mail stating that they can't believe what you did in this video. If the user clicks to view the video, a message pops up stating that they need to download some additional software to view the video. Once this has been clicked, malicious software (malware) is downloaded and run and your PC becomes the newest member of some bad guy's botnet.
This is a type of social engineering that makes you click on something and is sort of a trojan (something malicious posing as some useful application). Patching your machine is probably your best defense that you can do for yourself. One tool you can use that checks a wide variety of software on your PC is one from Secunia. Click here to scan your PC to see if you have any vulnerabilities that need patching.
Take care and have a fabulous Monday. I know I will.