In its security bulletin summary for April 2006 Microsoft reports three new critical vulnerability in its operating systems / applications, as well as one important vulnerabilitiy.

Critical is Microsoft’s highest vulnerability rating.

A summary describing briefly the vulnerabilities is available from Microsoft’s Security Bulletin Summary for April 2006.
From this page you will also find links to more detailed information in Microsoft's Security Bulletins MS06-013, MS06-014 and MS06-015 (critical), and MS06-016 (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.