I have been having this same problem as well. It started about a week ago.

I can play an mp3 in Windows Media Player for about 10 seconds, then I get a
popup telling me to download a file from flashcodec.com. The popup says:

You have chosen to open
Windows_Media_Player_Flash_Codec_Plugin.exe
which is a: Application
from: http://www.flashcodec.com

I also saved the file to scan it and the file appears to be:
trojan.downloader.wma.wimad.p

I can find many ways to remove the trojan downloader wma wimad (which I
don't have because I don't run the downloaded exe), but I can't find how to
remove the popup that is prompting me when I play any mp3.

I have run Windows OneCare, Windows Defender, PCTools Spyware Doctor,
AdAware, BitDefender, Panda Scan and HiJackThis. None of them can find what
is causing my mp3's to cause the popup.

When I try to play the mp3's in Winamp I get the sound of a scratched CD or
they do not play at all, but no popup. On a side not I can play wav files
just fine. I have looked all over the internet for the solution or removal of
this problem but I can not find one.

Please help! :)