Windows XP Service Pack 3 (SP3), just out from Microsoft as Release Candidate 2 (RC2), may not necessarily be worth the download now, but when it's finally released, it'll be worth the effort. It won't make any noticeable cosmetic changes to your operating system, but underneath the hood are several security improvements worth having. In addition, one researcher claims it will slightly boost XP's speed as well.

Microsoft is using an interesting method to distribute XP SP3. You don't download it as a separate .exe file - instead, you download a small executable file that, when run, changes your Registry so that Windows Update will download XP SP3. If you are comfortable with editing the Registry, you can instead use a hack to tell your Registry to download SP3. Here's how it works.

First, create the following new Registry key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate\XPSP3

Then create this string value for the new key:

RCPreview

with this value:

1c667073-b87f-4f52-a479-98c85711d869

The actual download of the Service Pack weighs in at 65.6 MB. Once I downloaded it, it took about 30 minutes to install on a 1.83GHz Core Duo laptop with 1GB of RAM. Reboot after the install, and you're in business.

Don't bother looking around for any visible changes to the operating system; you won't find any. You'll have to go to System Properties and check your OS version just to make sure it installed properly. Microsoft claims there is one, exceptionally minor interface change - according to the Release Notes, a Security Options Control Panel applet "offers more descriptive text to explain the settings and prevent incorrect configuration of settings." Doesn't make the pulse race, does it? But I couldn't find even that - my version of XP doesn't seem to have the applet in question. If you install a copy of SP3, and manage to track down the screen, please add a comment to this story letting me know how to get there.

One of the biggest changes to XP won't affect people who already have the operating system - it's to do with product activation and Genuine Advantage, Microsoft's way of validating its OS. Before SP3, when you installed XP for the first time, you had to enter a valid product key or else you couldn't install the operating system. With SP3, you'll be able to install XP for 30 days without the key. If you already have XP, of course, you won't need a product key to upgrade to SP3, so it won't really affect upgraders.

Security changes

Most of what's new has to do with security. In fact, five of the seven changes Microsoft lists on its release notes are security-related.

Particularly notable is network access protection (NAP), which is available on Vista and on Windows Server 2008, but until now was not available on XP. NAP gives network administrators a way to determine a computer's access to network resources based on the PC's identity, and whether the PC complies with security policies set by the administrator. With NAP, network administrators can set this kind of access at a finely granular level. In addition, NAP gives the administrator tools to bring the PC into compliance with the security policies, and then give the PC access to the network.

In addition, Microsoft has beefed up security by upgrading the random number generator. Last November, Israeli researchers said that attackers could exploit a weakness in Windows' pseudo-random number generator and be able to predict encryption keys. In SP3, Microsoft claims that flaw is fixed.

SP3 also rolls up a variety of previous patches and hot-fixes. Particularly noteworthy for Wi-Fi users is Wi-Fi Protected Access 2 (WPA2), which supports the new Wi-Fi Alliance certification for secure wireless networks. WPA2 is already available in Windows Vista and Windows Server 2003 with SP2.

A performance boost?

Microsoft did not claim any performance boosts for SP3, but one firm, after testing an earlier version of SP3, claims that in fact, users of Microsoft Office will see a moderate speed boost.

Using a previous version of SP3, Devil Mountain Software, ran its OfficeBench suite performance tests pre-SP3 and post-SP3 on an Office 2007-equipped notebook with a 2.0-GHz Intel Core 2 Duo processor and 1GB of memory. The company found a 10-percent speed increase compared to the pre-SP3 version of XP, which was equipped with XP SP2.

Craig Barth, Devil Mountain's chief technology officer, wrote in his company's blog, "Since SP3 was supposed to be mostly a bug-fix/patch consolidation release - unlike w/Vista SP1, Microsoft made no promises of improved performance for XP - the unexpected speed boost comes as a nice bonus."

On my test machine - the aforementioned laptop - I didn't notice a perceptible difference in performance between Microsoft Office 2007 pre-SP3 and post-SP3. But I'm not sure that a 10-percent difference would be noticeable.

The bottom line? If you're an IT pro and need to test out SP3 before its official launch, it's worth trying out now. Others might as well wait for the final version - there's nothing that's an absolute must-have right now, but the improved security will be worth it in the long run.