WASHINGTON — Microsoft, whose popular Windows software is a frequent target for Internet viruses, is offering a free security program to remove the most dangerous infections from computers.

The program, with monthly updates, is a step toward plans by Microsoft to sell full-blown antivirus software later this year.

Microsoft said yesterday that consumers can download the new security program from the company's Web site — www.microsoft.com — and that free, updated versions will be offered automatically each month.

It will be available starting Tuesday.

Also, Microsoft offered yesterday a free program to remove "spyware," a category of irritating programs that secretly monitor the activities of Internet users and can cause sluggish computer performance or popup ads.

Microsoft said the virus-removal program will not prevent computer infections and was never intended to replace the need for traditional antivirus software, such as flagship products from McAfee or Symantec.

But a senior Microsoft executive confirmed the company's plans to sell its own antivirus software, which would compete against programs from McAfee, Symantec and others.

Microsoft purchased a Romanian antivirus firm, GeCAD Software, for an undisclosed amount in 2003. Industry rivals expect Microsoft's formal entry into the market as early as the spring.