In its security bulletin summary for April 2011 Microsoft has published nine updates for critical and eight updates for important 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 April 2011.
From this page you will also find links to more detailed information in Microsoft's Security Bulletins MS11-018- MS11-034.
The critical updates address the following issues:
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 relevant security updates as soon as possible, to be protected from potential exploits.