Confused and angry users are blasting Microsoft on its own Vista blog over the requirements for downloading Service Pack 1 (SP1). And an industry analyst feels their pain.

Last week, Microsoft released Vista SP1 to Windows Update, giving most users their first shot at obtaining the service pack. Previously, only earlier testers, volume licensing customers and IT professionals and developers who subscribed to TechNet or Microsoft Developer Network had access to SP1.

But as it added Vista SP1 to Windows Update, Microsoft also spelled out numerous caveats, telling users that there are as many as eight different reasons why they might not find the update in the Windows Update listing on their PC. Among those reasons: any of 31 language packs, earlier installed versions of SP1, various prerequisite updates and a number of device drivers.

The service pack is being withheld from machines containing one or more of the listed drivers, because, as Microsoft put it in a support document, "these device drivers are problematic on Windows Vista-based computers when you update to Windows Vista SP1."

That made an industry analyst wonder about Microsoft's driver testing process. "When Microsoft said there were problems with drivers, I assumed it was some odd scanner or camera, or an ancient printer or something," said Michael Cherry, analyst with Directions on Microsoft, a research firm. "But then I saw the list. It makes me wonder what's going on with device driver testing.

"Microsoft keeps saying that there's this vast ecosystem of device drivers, but it appears there's a much smaller number of reliable, well-tested drivers. Because if these drivers [on Microsoft's list] were tested, that calls into question the testing process."

In fact, many of the complaints posted in comments to Microsoft's Vista blog were related to drivers. For example, one user tried to plumb the depths of his PC to determine why Windows Update suppressed the service pack, but gave up.