Microsoft Corp. on Tuesday released a dozen security bulletins and fixes for software including Windows and security programs but none for the latest Vista version of the operating system.

The advisories — half of them carrying the company's most severe "critical" rating — could let an attacker hijack a computer with little or no action from a user.

Some of Microsoft's security products such as Windows Live OneCare and Windows Defender are among the tools affected by the critical vulnerabilities along with Internet Explorer, MS Office and various versions of Windows XP, 2000 and Server 2003.

Windows Vista is not directly affected but the Defender anti-spyware tool ships with the new version of the operating system so it is possible users could be exposed to a threat that way.

Most of the security holes can only expose people's computers to risk if they click on a link to a specially-crafted website or open a document rigged to exploit a vulnerability.

Windows users who have activated the software's automatic update feature will receive the security patches on their computers without making any effort. The fixes can also be downloaded directly from Microsoft's website.