Microsoft has announced the upcoming release of 12 security updates, nine of which are for Windows.

One of the two patches for Office is rated "critical", while the one for Exchange has an "important" rating.

The company is also releasing a non-security update for Windows and two for Windows Software, all rated "high priority".

The "critical" rating is reserved for flaws that could allow an Internet worm to replicate without the computer user's knowledge.

The software giant has also announced that it will no longer patch a critical security vulnerability in its Windows 98 and Millennium Edition operating systems.

When an update for other Microsoft products to fix the bug was released on 11 April, the company promised that a patch was forthcoming for the older software.

However, the company has now issued a bulletin that states: "After extensive investigation, Microsoft has found that it is not feasible to make the extensive changes necessary…to eliminate the vulnerability".

"We have found that these architectures will not support a fix for this issue now or in the future."

Support for Windows 98 and Millennium Edition is set to expire within a few months.

Security patches are available for download at