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Once Microsoft saw how the new testing protocols were bearing out in the real world, they decided it might not be a bad idea to back-port many of those lessons plus other things gleaned along the way into XP itself. XP was and still is Microsofts most broadly-used incarnation of Windows, and it would be unwise at best not to apply to XP what theyd learned while developing Vista. Hence, the long wait for SP3, while many of XPs components have been revised to include ingredients that have incubated, so to speak, in Vistas development process.
Microsoft has gone on record to say that Vista SP1 isnt going to introduce much new user functionality into Vista, but XP SP3 is bringing in a bunch of new features for XP back-ports of things that have already figured into Vista.
Some of these features are under-the-hood, like the new Network Access Protection system used in Vista. Others are slightly more cosmetic but still useful, like the way new installations of XP do not have to have a product key entered at install time. There are still plenty of kernel-level things that are exclusive to Vista, like the I/O prioritization system, which arent likely to ever be back-ported to XP, and while they dont sound like things that would make users break down the doors to get Vista, they prove themselves in hands-on use.
It makes sense to add that much value back into XP when Vista itself has had a rocky start. Driver support for the 64-bit edition was spotty. People groused about the raised system requirements (although, in all fairness, seven years had passed since XPs release and the baseline for a new system had risen considerably). Early adopters encountered a spate of problems, as early adopters are wont to do, from the TrustedInstaller CPU-hogging issue, to the way Explorer would hang or lag horribly when copying large amounts of files. The CPU issue is still something of a pest, although less so on multi-core systems; the IE issue has already been fixed to a great degree by an interim patch.
Still, all that said, its about as rocky a start as XP itself had. And in that light, it makes sense for Microsoft to give XP that much more of a lease on life. It can only do them, and everyone else, good in the long run: its that much more goodwill from the users and they can never have too much of that and it generates that much more real-world experience they can then in turn re-apply to Windows as a whole. Will it put a dent in Vista sales? Maybe, but people seem to forget that, whether or not you choose XP or choose Vista, youve still chosen Microsoft.