Service packs are like "The Blob." Without any superpowers, you can't stop the delivery of the blob. And every couple of years or so, the blob comes back, in a newer, bigger form !
Now, you might like what the blob offers. For sure, the blob offers a lot of fixes.
But it offers them all at once.
And that might be good.
Or it might not be what you want at all.
Maybe you haven't had sufficient time to test the blob. Maybe you want to go blob-less because you're doing some other massive XP to Vista project or something.
If you've got WSUS or SMS / SCCM, then you're pre-protected from the blob, because you have to manually decide to push the blob out.
But if you're using good ol' Windows Update for your clients, you might want to run Screaming out of the Theater and grab your fire extinguishers. So, if, like in the movie, you're looking to freeze the blob, there are several ways (all contained within one download), and, of course, a Group Policy way to do it. Microsoft has a download just for you! It is called the Windows Service Pack Blocker Tool Kit... Isn't that original?
And inside, you'll find an ADM file called NoSPUpdate.ADM.
And inside that one ADM file, you'll be adding one additional setting to GP.
It's called: "Do not allow delivery of the service Pack through Windows Update or Automatic Updates."
Again, if you're using WSUS or SMS/SCCM the blob can't come and get you. You've got a blob-free zone. It's only for those of us without our Tin Foil hats on.
PS: Not too far away from me in Philadelphia is where they filmed "The Blob" with the famous scene of everyone running out of the theater. Want to re-enact? It's 9.00 PM on Fri July 11th in Phoenixville, PA. Yep, you can run out of the theater during the Blobfest 2008 reenactment. And a scream contest and a tin-foil hat contest. And, its just one day after the XP/SP3 blob is scheduled to come and get you. (You knew there was a tie-in somewhere, right?)