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.

While a firewall would block an SMB attack from the Internet, someone who controlled a machine within the corporate network could exploit this flaw to get access to another computer in what's known as an SMB relay attack. "I would label this as critical on a corporate network," he said.

To make matters worse, the SMB flaw was already publicly disclosed prior to Tuesday's updates, Microsoft said.

With just two updates, this is one of the quieter patch releases Microsoft has had this year. But there was some excitement at the end of October when Microsoft took the unusual step of issuing an emergency patch for a bug in the Windows Server service.

Microsoft had spotted this flaw being used in a small number of targeted attacks, and the bug was considered so serious that Microsoft decided to rush out the early patch ahead of Tuesday's regularly scheduled security updates. This flaw has not been used in widespread attacks, however, security vendors say.