In its security bulletin summary for September 2008 Microsoft has published four updates for new critical vulnerabilities in its operating systems / applications.

Critical is Microsoft’s highest vulnerability rating.

A summary describing briefly the vulnerabilities is available from Microsoft’s Security Bulletin Summary for September 2008.
From this page you will also find links to more detailed information in Microsoft's Security Bulletins MS08-052 - MS08-055.

The critical updates address the following issues:

• Several privately reported vulnerabilites in the Microsoft Windows GDI+.
• One privately reported vulnerability in Microsoft Media Encoder 9 Series.
• One privately reported vulnerability in Microsoft Office.
• One privately reported vulnerability in Microsoft Media Player.

Updates that fix the vulnerabilities are available from Windows automatic update mechanism for systems that support this. Alternatively, one may download updates from http://windowsupdate.microsoft.com.  

Norman advices all affected users to download the security updates as soon as possible, to be protected from potential exploits.