Microsoft released two security updates for its Windows operating system Tuesday to patch flaws that could give attackers new ways to install malicious software on a victim's computer.

The MS08-069 update fixes critical flaws in the Microsoft XML Core Services used by Internet Explorer and other programs to render Web pages. The second MS08-068 update fixes a less-critical bug in the Windows Server Message Block (SMB) software used by Windows to share files and print documents over a network.

Hackers routinely use Web bugs such as these XML flaws to infect Windows machines. "Anytime Microsoft updates Web vulnerabilities they're going to rate them as critical," said Andrew Storms, director of security operations with security vendor nCircle. For a Web-based attack to work, the victim must first visit a compromised Web page or open an e-mail that displays the malicious code.

Microsoft rates the SMB update as "important" for Windows XP, 2000 and Server 2003 users, and only "moderate" on Vista and Server 2008. But enterprise users should still take it very seriously, said Eric Schultze, chief technology officer at Shavlik Technologies.

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