Take your hands off that mouse. I know, you're ready to grab XP SP3. But slow down a sec and read my quick tutorial. It could save you some time and make your life with the new Service Pack less harrowing. (Tomorrow I'll tell you about my Windows experience with SP3 installed.)
Denny Arar posted Windows XP SP3 Available for Download late yesterday and she supplied links to the download. But before you do the deed, I suggest you do a little reading.
Wait a minute! You say you haven't followed the XP SP3 saga? Read Robert Strohmeyer's Windows XP SP3: First Impressions and Early Tests Say SP3 Speeds Windows XP. (I'll get back to the speed issue from that last report tomorrow.)
It's a Good Idea to Read This
Start with Microsoft's Steps to take before you install Windows XP Service Pack 3, a handy set of instructions to ensure a pleasant experience. It talks about "fast user switching" issues (like making sure everyone's logged off), administrator rights, and error messages you may encounter.
Then take a quick look at the Service Pack's Release Notes. Among other things, you'll discover that if you installed Internet Explorer 7 (or the IE 8 beta) before installing SP3, you won't be able to remove IE -- unless you first uninstall SP3. It's not an issue for most users, but it may be for you. If it is, be sure to uninstall IE7 or IE8 before installing SP3.
Update: There's a work-around for the IE 7 hassle. It's explained here. [Thanks, Jeff F.]
Update: You might also swing by Microsoft's Windows XP TechCenter.
Downloading vs. Windows Update
Downloading the entire Service Pack from Microsoft's Download Center, all 343MB of it, will take 30 minutes or longer, depending on your DSL or cable modem connection. (Think about the poor schlemiel -- or is he a schlimazel? -- who's still using a 33K modem...)
SP3 is big because it contains every known XP update in the universe. So unless you want to burn SP3 onto a CD, the best bet is to do the upgrade from Windows Update.
Update: You can grab SP3 on an ISO file to burn on a CD at Microsoft's Download Center.
You can do that by using any current version of IE and heading to Windows Update. Even easier, go to Start, Help and Support, and select "Keep your computer up-to-date with Windows Update."
The installer will check to see what you already have on your PC, so the download's sure to be smaller than the full-blown Service Pack -- at least it was on my PC.

Windows Update makes for a quicker SP3 installation.
I used Windows Update, and from start to finish -- downloading everything, lots of file backups and copying, and an automatic installation -- it took 40 minutes at my cable modem speed of 1.5 megabytes per second.
Tomorrow: SP3 is up and running. And I have news for you!
Why is one 344mb and the iso 544mb
Go to Microsoft Communities Newsgroups where you will finf numerous posts requesting help for the many problems caused by SP3.
Vista, the impending withdrawal of XP, and now SP3 will add to sales of the Mac.