So an EBud just sent you an E-Card greeting, supposed to be soo cute. And you play it, and you get a notice
Your player needs an extra codec. Shall we find it for you?
STOP! Right now, it's time to close the browser, flush your cache, and restart the computer.
Sometimes, you may see the above advice when surfing to various web sites that provide free movies (and no, we won't discuss what type of movies, though this problem is more common on certain types). It's the same thing though. The content (E-Card, online movies, whatever) is the attraction. The trojans are the payload. It's all just another way of hacking your computer.
Lavasoft News Beware - Desktop Hijacks on the Rise Again says
Watch out for the Zlob Trojan that poses as a codec needed to view a video, then installs a fake virus and urges its victims to download a rogue anti-spyware program to remove it.
And again we see the repeated advice
...even a spoofed e-mail that claims to be Windows Update (Microsoft never sends updates via e-mail)
as if I haven't harangued you about that before.
The best prevention is avoidance. Stay safe.