A day after Microsoft announced that it has finished work on the first major update to its Windows Vista operating system, the company conceded that many users won't have access to the software any time soon due to driver incompatibilities.

"We're still in the process of reaching out to the specific hardware partners that are affected and are providing them with the necessary guidance to ensure a smooth installation," a Microsoft spokesman said Tuesday.

In other words, Microsoft has yet to finish ensuring that Windows Vista Service Pack 1 will work properly with keyboards, printers, mice, and other peripheral devices commonly attached to personal computers.

Such peripherals use bits of software called drivers to communicate with the host computer.

The problem is that Vista SP1 won't install some device drivers correctly. Microsoft says the issue is confined to "a small number" of drivers. However, the company is refusing to say which drivers -- or which hardware manufacturers -- are affected by the trouble. "We have no additional information at this time," the spokesman said.

It's all leading to a chaotic roll out schedule for Vista SP1, which includes more than 300 hot fixes and is designed to improve Vista's performance in areas ranging from data protection to video display.

Microsoft on Monday said Vista SP1 has been "released to manufacturing."

But the software won't be available as a manual download until mid-March on Microsoft's Windows Update service. Even then, installation will be rejected on PCs with the problem drivers. Affected users who want to access the software regardless will be able to download it from Microsoft.com.

Microsoft will automatically push Vista SP1 to customers who have signed for automated downloads from Windows Update starting in mid-April. Again, however, PCs that are using the incompatible drivers will be blocked from receiving the update.

Windows Update will automatically install new drivers that fix the issue "as they become available" -- only after which will Vista SP1 be installed, according to a blog post Monday by Windows product manager Mike Nash.

The driver hitch also means that it may be months before Windows Vista SP1 is available as pre-installed software on new PCs or as packaged software in stores. "Both will be available in stores for new Windows Vista customers in the coming months," Nash wrote.

Some users responding to Nash's blog post expressed frustration about the delays. "This has got to be one of the worst excuses I have heard for announcing [the release] of a product and then telling your paying customers they must now wait six weeks to acquire it," wrote a poster going by the name 'Ntpro'.

Another poster, 'Colinbo', said the driver problem "sounds like a thinly veiled delay to me."