Symantec is warning people who routinely download media files such as .asf, .mp2, .mp3, .wma and .wmv files from random websites or P2P services such as BitTorrent, that these files might be infected with a trojan horse known as Trojan.Brisv.A.
If you've ended up with this virus on your PC, two things will happen.
First, the virus will look for the media files listed above and infect them with special code.
Second, when a user attempts to play one of these files using Windows Media Player, the special code attempts to open a URL which contains additional malware.
It's not clear what this malware could be or what it does, so it's probably best to avoid the problem altogether. If you've got anti-virus protection on your machine with up to date definition files, you're more than likely safe from this threat. If you don't, or you think you may already have the virus, you can download a free removal tool from Symantec.