They include one that could allow hackers to take over computers running the company's instant messaging programs.

One of the flaws carries the company's most severe "critical" rating, but it only applies to the older Windows 2000 operating system.

To be affected, a Microsoft security program manager says users would have to visit a Web site and install a program that could then run malicious code on their computers.

The other security vulnerabilities -- including the one affecting MSN Messenger and Windows Live Messenger -- were assigned the second-highest "important" rating.

The IM flaw would allow hackers to run malicious code on computers if users click on an instant message link inviting them to check out a video.

Analysts say hackers are beginning to target instant messaging programs because it's getting harder to trick people into clicking on links sent in e-mails.