REDMOND, Wash. — Microsoft Corp. released three security bulletins Tuesday to fix flaws that could let an attacker take complete control over a computer system.

The "critical" security flaws, the company's highest threat level, affect Microsoft's Windows operating system. The software maker released other patches to address less serious problems with Windows, the Exchange server system, services for the Unix operating system, Microsoft's Interactive Training software for Windows, and ISA server, a network firewall program.

Microsoft said it was rereleasing three previous security bulletins: one for Windows, one for the .NET framework, and one for an SQL server scanning tool.

Some of the flaws affect computers with operating systems dating back to Windows 98.

Users who have installed Microsoft's massive security update for Windows XP, Service Pack 2, also will need to install some patches unless they've signed up for automatic updates.