Microsoft have issued a critical update for Windows that fixes a vulnerabilities in Windows that could allow remote code execution. The fix is for XP, Windows Server, Vista and is classed as important for Windows Media Center TV Pack, Vista and Windows 7. It looks like the risk is in Windows Media Player and you can download the fix or wait for it to come over Windows Update
More details on Microsoft Technet blog
Microsoft Security Bulletin MS12-004 – Critical Vulnerabilities in Windows Media Could Allow Remote Code Execution (2636391)
This security update resolves two privately reported vulnerabilities in Microsoft Windows. The vulnerabilities could allow remote code execution if a user opens a specially crafted media file. An attacker who successfully exploited the vulnerabilities could gain the same user rights as the local user. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights.
This security update is rated Critical for all supported editions of Windows XP, Windows Server 2003, Windows Vista, and Windows Server 2008; and Important for all supported editions of Windows Media Center TV Pack for Windows Vista, Windows 7, and Windows Server 2008 R2. For more information, see the subsection, Affected and Non-Affected Software, in this section.
The security update addresses the vulnerabilities by correcting the way that Windows Media Player handles specially crafted MIDI files and the way that DirectShow parses media files. For more information about the vulnerabilities, see the Frequently Asked Questions (FAQ) subsection for the specific vulnerability entry under the next section, Vulnerability Information.
Recommendation. The majority of customers have automatic updating enabled and will not need to take any action because this security update will be downloaded and installed automatically. Customers who have not enabled automatic updating need to check for updates and install this update manually. For information about specific configuration options in automatic updating, see Microsoft Knowledge Base Article 294871.
For administrators and enterprise installations, or end users who want to install this security update manually, Microsoft recommends that customers apply the update immediately using update management software, or by checking for updates using the Microsoft Update service.