But some experts think that the problem must be related to under-the-hood changes in Vista SP1, especially those concerning security and user privileges, which can directly affect how applications and drivers are installed.

"Microsoft looks like they are monkeying around with a lot of lower-level stuff," said Paul Morris, a project manager at QualityLogic, a Moorpark, Calif., software and driver testing firm. "In my mind, you've got to treat SP1 as an entirely new OS."

Vista's overhauled security model had two major changes. First, local users who were formerly granted administrator access by default are granted standard access in Vista. Second, a new feature called User Account Control, or UAC, prompts users to verify whenever a new application or driver is about to be installed.

Both moves help to prevent viruses and malware from taking hold. But UAC has been criticized by some users for its overzealousness, though UAC pop-ups decrease dramatically after the first month, according to Redmond.

Moreover, moving users to standard rather than administrator accounts wreaked havoc on many applications and drivers that depended on full administrator access to install or run properly.

If SP1 makes even small changes to Vista's security model, that could cause drivers to break again, according to Ian Lao, an analyst at In-Stat. Why? For one, most vendors took the easier, less rigorous option of patching and modifying their drivers to make them Vista compatible, rather than "rewriting them from the ground up," said Lao. That makes them more fragile.

Moreover, many drivers, unlike applications, can't be completely shut down during an installation process. The upgraded software -- in this case, Vista -- may not be able to properly update the drivers, he said. Thus, permissions may be lost during the SP1 upgrade, or resources moved to different locations.

Of drivers and ass-kickings
Drivers were a major bane to Vista in its early months last year.

Despite Microsoft's claim that more than 1.6 million peripherals worked with Vista at its launch, there were numerous complaints from early adopters, gamers, and other power users about nonworking devices.

"It definitely wasn't just a vocal minority that had a problem with drivers at Vista's launch," said Robert McLaws, a blogger who also tested SP1 but didn't face any driver issues.

Indeed, driver issues had been on the wane. At its Windows Hardware Engineering Conference (WinHEC) conference last May, Microsoft said that 1.9 million devices supported Vista.

Microsoft has not updated that number since then. But it says that more than 78,000 hardware products are now supported by Windows Update, up from 34,000 at launch.

And 17,000 peripherals and devices have passed one of Microsoft's two rigorous certification programs: Certified for Windows Vista, which requires successful testing by a third party such as QualityLogic, and the more self-reported Works with Windows Vista program, which is more reliant on self-reporting.

"A lot of OEMs did not take the Vista launch seriously," McLaws said. "So Microsoft took their problem reports to vendors and kicked some ass."

In a PowerPoint presentation distributed to media last month, Microsoft was confident enough to state, "Many device and software problems behind us: Nearly all Windows Vista PCs have drivers for every single device installed, available on WU [Windows Update] or from vendor websites."

But Microsoft may have relaxed too soon. Morris says none of vendors that came to QualityLogic for certification of their Vista drivers have returned seeking the same service for SP1.

"They may be hammering on them tooth and nail internally, but they're not asking us to touch it," he said.

The result? Monday's delay.

Gregg Keizer contributed to this story.