Could my computer get infected with any harmful software if I watch something online?

[Muckdoomee]

No, you will need to install a fake codec or a fake flash update in order to get malware. It is by user intervention.

If you come across a video clip on the net and you want to watch it, and then an error message pops up saying, sorry you cannot watch it, you need to install a flash update to watch it, that fake flash update is malware. Now, this sort of attacks are prevalent on social networking such as MyFace, FaceBook, AIM, MSN and more.

This practice of installing fake codec or flash update has been around for a while, and now, there are more and more attacks on social networking where the users are more likely to drop their guard if a video clip was sent from their "Buddy" but the video clip was never sent from their "Buddy" in the first place.

There are also fake YouTube pages too with the same principle of asking you to install fake flash update, again, user intervention is involved. Just google Koobface, fake codecs, fake flash updates and fake youTube pages and loads of sites that can tell you more on there.