The Sunbelt Blog has a good reminder post as to the dangers of being duped into installing fake codecs (compressor/decompressor) for multimedia applications. The codec of concern here is called Emcodec.
[tag]Emcodec[/tag] makes a lot of claims (40% better "quality", playback support for all popular players ...) but it's really just a front for the Trojan.Zlob.J Trojan. It's yet another example of how malware is being disguised and legitimate tools and as browser holes are closed up you can expect more and more of this kind of behavior in the future. Another one of these fake codecs is called Vcodec. Again it hides a Trojan behind bold claims.
Apart from running an up-to-date antivirus program (all the top AV apps will detect this) it's also a good idea to approach any download with skepticism. Do a search and see what other people are saying about the download and remember the golden rule - if in doubt, DON'T!