Microsoft will limit the number of downloads for the beta of Windows Security Essentials, its new free anti-virus software, when it posts the program on Tuesday.

Downloads will be capped at 75,000, a Microsoft spokeswoman said on Monday.

Windows Security Essentials, the replacement for Windows Live OneCare, the for-a-fee security package that Microsoft is ditching on June 30, will be available for download at approximately noon, Eastern time (5pm BST), the spokeswoman added. The free download will be posted on a new Microsoft site dedicated to Security Essentials.

Microsoft has pitched the software as a basic anti-virus, anti-spyware product that consumes less memory and disk space than commercial security suites like those from vendors such as Symantec and McAfee, and so is suitable for even low-powered PCs such as netbooks.

"Netbooks are one focus of Windows Security Essentials," said Alan Packer, the general manager of Microsoft's anti-malware team, in an interview last week. "We have tested it on netbooks, and a gigabyte [of memory] is actually plenty."

He conceded that on systems with slower processors and limited RAM, however, the software will hit performance, especially when users run multiple applications at the same time. "I don't want to oversell here," Packer said. "There's definitely an impact on netbooks, and although we're trying to minimise [that], you're going to notice it's there."

Consumers won't be the only ones eager to queue up for a copy of Security Essentials. Anti-virus vendors will want an early look to match it against their wares, free or not, as will others in the Windows ecosystem.

That group includes hackers, said David Bookbinder, the owner of Total PC Support, a Peabody, Mass.-based PC computer support firm, who doesn't hold Microsoft's security tools in much esteem.

"Microsoft has already repeatedly shown a particular incompetence when it comes to identifying and preventing malware, and I predict this new application will fare no better," said Bookbinder. "The malware writers, I am sure, will poke through this like a rat into Swiss cheese."

His recommendation to Microsoft? "Just stay out of the anti-malware business ... [and] tear down all the useless layers of Windows where the garbage hides."

Microsoft has had bad luck in the past when it has limited the number of downloads for previews of its software. Last January, the company had to restart the launch of Windows 7 Beta after its servers were overwhelmed because users, who had been told Microsoft would cap the downloads, rushed to grab a copy.

In May, when Microsoft offered Windows 7 Release Candidate (RC), it made it clear to users that it wouldn't restrict the number of copies downloaded from its site.