In its security bulletin summary for May 2007 Microsoft reports a total of seven 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 May 2007.
From this page you will also find links to more detailed information in Microsoft's Security Bulletins MS07-023 - MS07-029.

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.