Microsoft Corp. Tuesday warned of three "critical" security flaws in its Windows operating system that could allow attackers to take control of a computer.

Microsoft, the world's top software company 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.

One of the critical vulnerabilities appears in some versions of Internet Explorer that could make it possible for an attacker to use the Web browser to take total control of a PC.

The other two "critical" vulnerabilities were in the Windows operating system.

The company defines a flaw as "critical" when the vulnerability could allow a damaging Internet worm to replicate without the user doing anything to the machine.

The company also issued another security warning it rated at its second-highest level of "important" for Microsoft Windows as well as one it gave a severity rating of "moderate."

A vulnerability defined as "important" is one where an outsider could break into a machine and gain access to confidential data but not replicate itself to other computers, Microsoft said.

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