Microsoft is facing an early crisis of confidence in the quality of its Windows Vista operating system as computer security researchers and hackers have begun to find potentially serious flaws in the system that was released to corporate customers late last month.

On Dec. 15, a Russian programmer posted a description of a flaw that makes it possible to increase a user\ufffds privileges on all of the company\ufffds recent operating systems, including Vista. And over the weekend a Silicon Valley computer security firm said it had notified Microsoft that it had also found that flaw, as well as five other vulnerabilities, including one serious error in the software code underlying the company\ufffds new Internet Explorer 7 browser.

The browser flaw is particularly troubling because it potentially means that Web users could become infected with malicious software simply by visiting a booby-trapped site. That would make it possible for an attacker to inject rogue software into the Vista-based computer, according to executives at Determina, a company based in Redwood City, Calif., that sells software intended to protect against operating system and other vulnerabilities.