Microsoft Corp. released four critical patches Tuesday to plug security holes in several versions of its Windows operating system, Internet Explorer Web browser and other programs.

The patches that carried Microsoft's highest security warning all are to prevent malicious hackers from remotely taking control of computers without permission.

Three of the patches aim to protect Windows users who unwittingly expose their computers to attack by visiting Web pages infected with malicious code, or look at similarly tainted e-mails with Outlook Express or Windows Mail. A fourth patch prevents hackers from gaining remote access to PCs by installing a specially crafted program.

Two of these critical updates fix holes in the company's newest operating system, Windows Vista, which Microsoft has touted as the most secure ever.

Vista went on sale to consumers at the end of January; in April, Microsoft broke its once-monthly update schedule with an emergency fix after Microsoft and security experts found that hackers were exploiting a hole in the way Vista and other versions of Windows handle animated cursor files.

Besides the critical fixes, Microsoft released a patch for its Visio program for making diagrams and a vulnerability in Windows that could allow unauthorized users to break into computers to steal passwords and other user information.

Microsoft also released seven non-security, high-priority updates Tuesday, including a monthly update to a tool that removes harmful software from PCs.