"This binary is then decrypted and executed as a malware detected as TROJ_DLOAD.QYUA," says Trend Micro's engineer. "We’re still conducting further analysis on TROJ_DLOAD.QYUA, but so far we’ve been seeing some serious payload, including rootkit capabilities."
All this happens without the victims being none the wiser - they simply see (and hear?) the MIDI file being played as they expect it to.
To protect themselves from such attacks, users of Windows XP SP2 and SP3, Server 2003 SP2, Vista SP2, and Server 2008 SP2 are advised to to patch their systems as soon as possible.