In its security bulletin summary for September 2006 Microsoft reports only one new critical vulnerability in its operating systems / applications, as well as one important, and one moderate .

Critical is Microsoft’s highest vulnerability rating.

A summary describing briefly the vulnerabilities is available from Microsoft’s Security Bulletin Summary for September 2006.
From this page you will also find links to more detailed information in Microsoft's Security Bulletins MS06-054(critical), MS06-052 (important) and MS06-053 (moderate).

The critical vulnerability is in Microsoft Publisher, which is part of Microsoft Office

Updates that fixes 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.