Tips on web browsing: Some web pages create fake virus alerts.
You're browsing the web for something as inane as a picture of a green algae, only to find that suddenly you're computer is 'compromised'. In fact, it's not compromised unless you allow it to actually download and install the trojan software pretending to be antivirus software. Learn how to recognize the signs of a web page trying to take over your computer.
UPDATE August 2012:
This video:
• Video
from Mastrqeef117 shows a similar attempt.
Be careful out there. :)