Microsoft is preparing to release six software updates on Tuesday, four of which will fix 'critical' vulnerabilities in Windows or its components. 

Two of the 'critical' bulletins address flaws in Windows XP, 2000 and Server 2003.

A flaw in versions 5.01, 6.0 and 7.0 of Internet Explorer spans all currently supported versions of Windows including Vista. It ranges in severity, however.

Users of all versions of the browser on all Windows versions including Vista should prepare for a 'critical' update. The flaw only received a severity rating of 'moderate' for systems running IE 7 on Windows Server 2003.

The fourth 'critical' flaw affects only the Mail application bundled with Vista. Although the vulnerability occurs on Outlook Express for older Windows versions, the flaw in this case received a severity rating ranging from 'important' to 'low'.

A security rating of 'critical' typically indicates that attackers can exploit the vulnerabilities without any user interaction.

The two non-critical security holes affect Vista and Visio 2002 and are respectively rated 'moderate' and 'important'.

June is the first month that Microsoft has published its expanded advanced security warnings. The previous system grouped flaws by affected application, and did not disclose the component or version of the operating system. 

The Advance Notification Service is designed to allow IT administrators to prepare for upcoming patch releases. The warnings are sent out on the Thursday before each patch release, scheduled for the second Tuesday of the month.