Microsoft has released a security update for Windows XP, Windows XP x64 Edition, and Windows Server 2003

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.

Security Update for Windows XP x64 Edition (KB2564958)
Security Update for Windows Server 2003 (KB2564958)
Security Update for Windows XP (KB2564958)