Microsoft has released an updated security fix for a vulnerability in Windows Media Player 11 on Windows XP X64 bit systems and it is available for download from the Microsoft Download Center and Windows Update.
A security issue has been identified that could allow an unauthenticated remote attacker to compromise your system and gain control over it. You can help protect your system by installing this update from Microsoft. After you install this update, you may have to restart your system.
You can read more at http://support.microsoft.com/kb/2378111.