WASHINGTON -- Microsoft on Tuesday released a dozen software updates to fix 16 security flaws -- half of which it deemed "critical" -- in all versions of the Windows operating system and in applications such as its Internet chat and media player products.

More than half the patches were intended to address security glitches found in Service Pack 2, the massive software security upgrade Microsoft made available to Windows XP users last August.

Security experts said one of the weaknesses in Windows disclosed Tuesday could be used to spread a computer virus. The flaw involves the "server message block" service in every version of Windows that allows users to share files on a network. Attackers could potentially exploit the weakness over the Internet if computer users fail to turn on their computer's firewall. Hackers could also exploit the flaw by tricking a user into clicking on a specially crafted web link in an e-mail.

"Out of all of the vulnerabilities, this one is the most likely to become the next widespread Internet worm," said Oliver Friedrichs, senior director of security response for Symantec, a California-based Internet security company.

Microsoft also issued a bundle of six fixes for vulnerabilities in its widely used Internet Explorer web browser. One flaw was recently exploited by "phishers," criminals who engage in identity theft by creating authentic-looking e-mail messages and web sites designed to lure people into disclosing financial data.