Microsoft Corp. released six new security bulletins for software that includes its Windows Vista operating system and Internet Explorer web browser Tuesday, including four with a "critical" top severity rating.

Three of the bulletins alerted users that an attacker could hijack a computer on which the software is running, while a fourth advised that information stored on an affected machine could be disclosed over the internet.

The 15 bug fixes affect versions of Windows, including the latest Vista edition, Internet Explorer, Office, Outlook Express and Windows Mail e-mail management software, and the Visio diagramming program.

Critical flaws repaired in the monthly security update commonly referred to as "Patch Tuesday" include the hijacking and information disclosure risks in Windows Vista and XP, Internet Explorer, Outlook Express, Windows Mail and Visio.

The software patches are available as a free download from Microsoft's website.