Microsoft has blamed "human error" after its Windows Genuine Advantage (WGA) validation service incorrectly labelled innocent Windows buyers as software pirates.

The software vendor also promised that changes are being made to avoid a repeat of the glitch, which affected users for nearly 20 hours a week ago. Users whose copies of Windows erroneously failed WGA's anti-piracy tests were prevented from downloading most software from Microsoft's website. And those with Vista had some features crippled.

Alex Kochis, Microsoft's senior WGA product manager, wrote in a blog posting that the troubles began after "preproduction code" was installed on live servers.

Those systems had yet to be upgraded with another code change designed to enable stronger encryption and decryption of product keys, Kochis added. As a result, "the production servers declined activation and validation requests that should have passed," he wrote.

A quick code roll-back fixed the problem within 30 minutes, according to Kochis. But it didn't reset the validation servers, which handle legitimacy checks on downloads and other transactions.

"We now realise that we didn't have the right monitoring in place to be sure the fixes had the intended effect," Kochis wrote. He also said that Microsoft is taking steps "such as increasing the speed of escalations and adding checkpoints before changes can be made to production servers."

Earlier this week, Microsoft said that fewer than 12,000 systems were affected worldwide. But users lit up the company's support forums with more than 450 messages about the howler.

"A system that's not totally reliable really should not be so punitive," said Gartner analyst Michael Silver.

Michael Cherry, an analyst at Directions on Microsoft said he was surprised that it was even possible to accidentally load the wrong code onto live servers. "It just begs the question of what other things have they not done?" Cherry said.