Various internet sources are reporting two Microsoft patch concerns -- a failed "real" patch released by Microsoft, and a third party assembling various hot-fixes, and calling them "Windows XP SP3 Preview Pack".
There is no official "Windows XP SP3"... the third party preparing them clearly mentions that on their website, but it is very tempting for people to go there, and grab the update, and install it. Windows is a flawed operating system that requires local administrator authority to run all of the software, so not even compaines can protect themselves at the local machine level from having users improperly installing these patches, and possibly further corrupting the computer.
Microsoft's Mike Brannigan wrote stating, "The hotfixes are not as rigorously tested at public released ones." "Installing all the 'privates' may make your machine LESS stable and will also put you out of support from Microsoft or an OEM as you are installing incorrectly issued private hotfixes."
But Mike's assertion that released hotfixes are 'rigorously tested' may be a misnomer too: Microsoft for the second time in three months has issued a critical buggy patch. Mike Reavey of Microsoft's Security Response Center wrote "Yes we are aware of some of the information floating around about problems after installing the MS05-051 update on Windows 2000 systems." Reported problems include problems with the Windows Firewall product, users seeing a blank screen after installing the patch, and other strange behaviors, especially on Windows 2000 systems.
This is why companies NEED to test patches with a small representative group of machines before the patch is widely implemented within a corporate environment -- if the patch causes systems to crash & burn, then only a handful or so of people are out of luck, instead of the whole corporate computing environment. And this is also why people putting together un-official patches together on websites is such a dangerous situation -- IT departments might not have any clue on what really exists on the corporate workstation.