Microsoft on Tuesday warned of five "critical" security flaws in its Windows operating system and Office software that could allow attackers to take control of a computer. Microsoft, whose Windows operating system runs on 90 percent of the world's computers, issued patches to fix the problems as part of its monthly security bulletin.

The world's biggest software maker defines a flaw as "critical" when the vulnerability could allow a damaging Internet worm to replicate without the user doing anything to the machine.

Two of the critical flaws related to Windows and the other three to earlier versions of Office. More recent versions of Office and two other bulletins for Windows received security warnings rated at the second-highest level of "important."

The company has been working for more than three years to improve the security and reliability of its software as more and more malicious software target weaknesses in Windows and other Microsoft software.

The latest patches can be downloaded at www.microsoft.com/security.