In its security bulletin summary for April 2007 Microsoft reports a total of five new critical vulnerabilities in its operating systems / applications, as well as one important one. One of the critical is the one where an update was published in Microsoft's unscheduled security update earlier this month.

Critical is Microsoft’s highest vulnerability rating.

A summary describing briefly the vulnerabilities is available from Microsoft’s Security Bulletin Summary for April 2007.
From this page you will also find links to more detailed information in Microsoft's Security Bulletins MS07-017 - MS07-021 (critical) and MS07-022 (important).

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.