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. (You can find the instructions on my blog.
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 1.83 GHz Core Duo laptop with 1 GB 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.