The average user with Vista installed on a home PC

"For the average user who doesn't want to be overly involved in the maintenance of their PC, our advice is to make sure that Windows Update is configured to automatically download updates and then to sit tight," Zipkin said. "And when the time is right, we're going to offer SP1 and your PC will pull it down. It will not install automatically, but you'll get a little notice saying 'SP1 has been downloaded. Click here to begin the install.'"

"We're going to start the process in mid-April," he said, adding that not everyone will get the SP1 code pushed to them right away because Microsoft tries to keep its resources for sending out software from being overloaded.

Once SP1 is downloaded, the installation process will be roughly one hour, Zipkin said.

The average user with Vista's predecessor, Windows XP, installed on a home PC, who is looking to get the most up-to-date version of Vista at retail.

"We've said conservatively that we'd have it in stores as early as April," Zipkin said. "But, as you can see, some retailers, such as Amazon, already have it for sale. And so people will be seeing this in stores quite soon."

Amazon's March 19 availability date is accurate, Zipkin said. (There had been some confusion about the date, which changed earlier this week.)

As for Vista SP1's availability elsewhere: "It comes down to the channel itself and the store itself. But we should be seeing them between now and early April," Zipkin said.

(It's worth noting that some people have had bad experiences putting Vista on older hardware. Microsoft has a tool to determine whether a given machine has enough horsepower to run Vista. And for those sticking with Windows XP, Zipkin provided no update on when Service Pack 3 is due -- still on track for the first half of this year.)

Someone shopping for a new PC now who wants to get a machine with the most up-to-date version of Vista.

Not surprisingly, Zipkin's advice is don't wait for new PCs with Vista SP1. Buy a PC now and get Vista SP1 via Windows Update (see below) or wait for the automatic update next month, he said. But if you do decide to hold off, the wait won't be very long.

"We expect that new PCs [with Vista SP1] will be appearing in channel, similar to retail, in early April although perhaps a little bit sooner," he said.

Unlike with the launch of Vista -- a marketing event coordinated to make the software available through all channels at the same time, Jan. 30, 2007 -- service pack releases are more fluid. And PC manufacturers have had the Vista SP1 code since early February.

John Gray, director of program management for Windows Sustained Engineering, explained.

"With service packs, it really just depends on how fast the individual PC makers and retail channels can build the product they need and move it through their inventory system and into the channel. There's no moratorium or restriction based on when they can and can't go live," he said.

The early adopter or technology enthusiast with Vista already installed on a PC

"He's going to want to get it today," Zipkin said. "This is the guy who likes to install things right away. He or she should visit Windows Update and click on 'check for updates.' And he may need to install some updates that are already there, but once SP1 appears, that's all he needs, he should go ahead and download and install it."

(To get to Windows Update from a Vista PC, hit the Start Menu, select All Programs and then select Windows Update.)

Some people, however, will not be able to get SP1 this way right now.

While Microsoft has made progress in getting more of the drivers that run devices such as printers and digital cameras compatible with Vista, compatibility for some devices regressed with test versions of SP1. In fact, Microsoft delayed releasing SP1 to the public until March because of these issues, Nick White, a Microsoft product manager, wrote on the company's Windows Vista blog.

"Windows Update will detect drivers that we know may be problematic when updating to SP1 and will not offer the service pack until an update has been installed," White wrote, adding that Microsoft has identified "a small number of device drivers that may be problematic after an update from Windows Vista to Windows Vista SP1," and is working with device vendors to get driver updates.

Another reason you might not be able to get SP1 via Windows Update today: It was released in English, French, Spanish, German and Japanese. People running Vista in one of the 31 other languages in which it is available have to wait until mid-April.