Microsoft Corp. said Thursday it plans to release a beta test of its free computer security program next week and is on track to launch a finished product in the fall.

The program, Microsoft Security Essentials, is designed to find and kill malicious software that can steal passwords and other personal information or turn PCs into spam distribution hubs.

Once the PC security software is installed, Microsoft said it will download updated lists of identified malware daily, but will keep a low profile unless it detects dangerous software.

Theresa Burch, a director on the security software team, said the program tries to spot malicious software even if it's not on the list of known corrupters. When it encounters something suspicious, it checks with a Microsoft server for updated intelligence before allowing the program to run, a process Burch said is almost instantaneous.

Microsoft also maintains a database of trusted software sources, so the tool won't accidentally block items like Google Inc.'s Web browser toolbar, she said.

Security Essentials will compete with rival subscription programs from McAfee Inc. and Symantec Corp., and with several other free packages. But Burch said Microsoft isn't going after those company's customers — instead, the goal is to improve security overall by getting people who don't have current antivirus software to protect their PCs.

In fact, it's important that Microsoft's competitors stay in business, Burch said.

"If there was only one solution out there addressing threats, all the malicious software developers out there would have a very easy target," she said.

The security software will come as a free download, but it won't be part of Microsoft's Windows 7 operating system, which goes on sale in October. Bundling the two could be fodder for antitrust complaints.

After the final version launches, Microsoft will discontinue its existing security program, the more robust $50 Windows Live OneCare.

In a statement Thursday, antivirus software maker Symantec objected to Microsoft's description of Security Essentials.

"Referring to Microsoft's basic antivirus and antispyware product as an essential security solution is misleading," said Dave Cole, a senior director at Symantec. He said PC users need the extra firewall protection, spam fighters and other features that come with subscription security programs.

"The freeware space is crowded and Microsoft is just joining the fray," Cole said.