REDMOND – Almost since the day Microsoft Corp. released its Windows XP computer operating system nearly three years ago, it has been a favorite target of hackers and critics eager to stress its numerous security shortcomings.

Now, more than two years after promising to do something about it, Microsoft is about to release the biggest update ever for Windows. The free upgrade is designed to make users safer from cyberattacks by sealing entries to viruses, better protecting personal data and fending off spyware.

Microsoft Chairman Bill Gates said the upgrade, dubbed Service Pack 2, revises less than 5 percent of the millions of lines of code that make up Windows XP – but adds more value than any update the company has ever done.

Some of the nearly $1 billion that’s gone into Service Pack 2 also will be used on future versions of Windows. But Gates said it was absolutely necessary to give away the security advances now because of the barrage of attacks plaguing Windows-based computers.

“If we weren’t viewing this as such a key priority, then we wouldn’t be giving it away as a free thing,” Gates said.

The long-awaited update is due to be completed “in the coming days,” said Microsoft senior product manager Matt Pilla.

Service Pack 2 has been delayed as programmers have worked to make sure the new security safeguards would not keep people’s favorite applications – such as online games and music download services – from working right. But such delays aren’t unusual in the software industry, and especially with such a massive undertaking as this.

For regular users, the most noticeable change will be a series of new prompts. The idea is that if users have to actively give permission for programs to interact with their computers, there is less chance they will be hit by a virus or inadvertently allow malicious software that can monitor computer activities.

It’s always risky to ask loyal customers to suddenly do things differently, but Gates said the changes aren’t major and are worth it.

“Believe me, a click – the time it takes to click vs. if you have a security problem, it’s a dramatic benefit,” he said.

Many security experts agree, arguing that the security changes are badly needed. Analyst Joe Wilcox of Jupiter Research likens the situation to the changes people might make when entering a bad neighborhood, which is what the Internet has become with rampant security attacks.

“Maybe you don’t go there at night (or) you lock the doors in the car. You change your behavior, and that’s what these things are going to do,” he said.

But analysts have raised concerns that if the system is too confusing or causes too many other applications to fail, then users will simply turn it off.

That’s one reason Microsoft delayed the system, to improve its compatibility with consumer applications.

Among other changes, the new system automatically turns on a Windows firewall to better guard against attempts to infiltrate personal computers. It also creates a “Windows Security Center” to help users monitor their security sentinels – including those from other companies – such as anti-virus protections.

Service Pack 2 also fortifies protections on the Internet Explorer browser and offers tougher policing against e-mail-borne attacks.

Gates conceded that he’s heard complaints that parts of the free update, such as its firewall, duplicate features others sell separately. Those companies now “need to move up to another level of innovation,” he said.