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The software wasn't supposed to be available until mid-March, however a glitch on Thursday meant that Zehr and some other general users were able to download the Vista update.

The update was not pushed out via Windows' Automatic Update feature, but was listed for owners running the 64-bit version of Vista who chose to "check for new updates" via Windows Update.

"A build of SP1 was posted to Windows Update and it was inadvertently made available to a broad group," Microsoft said in a statement. "The build was intended only for our more technically advanced testers, and was meant to only be offered to those with a specific registry key set on their PC. For general availability, we are still planning to make SP1 broadly available in the mid-March timeframe."

Zehr, who works for a technology leasing company in Illinois, said the upgrade took less than an hour and there were no immediately apparent hiccups.

"Unfortunately, I haven't had enough time on the machine to tell if anything is really better or broken, but I've always had an image backup just in case," Zehr said in an e-mail interview. "So far so good."

Zehr has two other Vista machines, including one other 64-bit system, but was unable to see the available upgrade for those PCs. He said that if the glitch had to happen to someone, he was glad it was him.

"I'm also a computer hobbyist, so Microsoft couldn't have picked a better guinea pig, glitch or no glitch," he said. Although he was not a beta tester for Vista, he did help test Windows XP just prior to its debut in 2001.

Microsoft finalized the code for SP1 earlier this month. Initially, the company planned to make all users wait until March for the update because of some driver-related issues that had yet to be resolved. However, the company eased its policy some, making it available earlier this month to businesses that have volume licensing pacts as well as to MSDN and TechNet developers.

The glitch is the second SP1-related issue for Microsoft in recent days. The company had to pull another update, this one a set of files necessary to move to SP1, because some users were sent into a repeated reboot cycle by the files.

Update: Microsoft revised earlier comments, confirming that those with automatic update settings configured in certain ways may have had the update pushed to them automatically.

A company representative also stated that the version pushed out is the final RTM version even though it may carry a beta designation.