Microsoft Corp. published six security fixes — half of them rated "critical" — in its regularly scheduled software update Tuesday.
The three patches with the company's most severe rating are designed to prevent malicious hackers from remotely taking control of computers without permission.
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One of the updates targets a vulnerability in the company's spreadsheet program, Excel, that could let hackers break into a PC if its user opened a tainted spreadsheet.
Microsoft also addressed critical holes in its .NET Framework, a massive library of computer code that is part of its newest operating system, Windows Vista, and versions of Windows XP, Windows 2000 and Windows Server 2003.
The third critical update is intended to prevent attacks on Windows 2000 Server and Windows Server 2003.
The company also released patches with less urgent security ratings for the firewall built into Windows Vista, and for Microsoft Office Publisher 2007 and Windows XP Professional.
PC users can visit Microsoft's security Web site to download updates, or sign up for an automatic security update program.