Microsoft released a number of critical and important security bulletins today. You can get information about these at the Security Bulletin Search site.  Here's a link to the bulletin summary for April 2005:

Technical Microsoft Security Bulletin Summary for April 2005

And this will take you to the consumer Security Updates page:

Consumer Security Update Page
Microsoft issues a security bulletin when a security update becomes available for Microsoft software. This page contains links to the summary announcements posted on Microsoft.com for widely released security updates, as well as the bulletin numbers for these releases. To get more technical information on the updates, follow the links in the summary announcements to the technical bulletins that are posted on TechNet.

As always, be sure to turn on Automatic Updates and visit Windows Update to keep your machines safe.