"It's an important, significant update to the operating system, and there are enough changes in some critical system files that users should back up their files," Sullivan said.

What you'll get

XP has been used by virus writers, worm authors and hackers like a digital punching bag, with consumers and businesses taking the blows.

Among Service Pack 2's features:

XP's Internet Connection Firewall now becomes Windows Firewall, and is turned on automatically. Previously, users had to enable the Internet-protection feature.

If you already are using a software firewall from a third-party maker, check with the company about Service Pack 2 compatibility issues. In most cases, conflicts arise when there is more than one software firewall in place.

Sullivan of Microsoft said that ZoneAlarm, for example, is updating its firewall software so it can turn off the Windows Firewall if a user prefers to keep ZoneAlarm instead.

There is a new Windows Security Center, a place for users to check their anti-virus and firewall software. The Automatic Updates feature is also located there.

Once you're connected to the Net, the Windows Security Center will let you know if updates are needed to security-related programs.

A major patch for the Internet Explorer browser corrects a critical flaw that can let hackers take control of individual computers.

Microsoft released a patch for Internet Explorer July 30, but if you haven't yet downloaded it, it is part of Service Pack 2.