Fake YouTube pages used to spread viruses
A program circulating online helps hackers build those fake pages. Users who follow an e-mail pointing them to one of the pages would see an error message that claims the video they want won't play without installing new software first. That error message includes a link the hacker has provided to a malicious program, which delivers a virus.
This has been happening for a while now, but it's still going on. The best thing to do is to NOT download the plugin/codec if a site tells you you need a special program or codec or plug-in to play the video from a site.
If you think you need special software, go to the actual site that distributes the real plug-ins needed. For most, there is Adobe Flash, Java, Quicktime (I recommend Quicktime Alternative) and Windows Media Player (here's the plug-in for Firefox).
If you are using Firefox 3, here is the link to download the Adobe Flash 10 beta (Windows Version). Version 10 works better for Firefox 3. And, sometimes you might need Real Player (I recommend the alternative version here).
Normally a site with a video embedded won't have a direct link to the above sites - the video space will be replaced with a notice that you need a plug-in to play the video. Fake sites will have direct links or pop-ups offering you a special plug-in. Don't be fooled. If you already have the plug-ins that I've mentioned above, and the video won't play, it is most likely fake so run away from the site.