Microsoft today announced the public availability of Service Pack 1 for Windows Vista -- the first big bundle of updates and fixes for the year-old operating system. The company says SP1 improves Windows Vista's performance and reliability. But in certain cases, it will still be a while before the service pack is actually installed and running on Vista machines.

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Here's a guide for anyone trying to figure out what's going on:

As of today, the company says Windows Vista SP1 can be retrieved and downloaded manually through the Windows Update feature inside the operating system or by using the Microsoft Download Center online site. See details in this post on the Windows Vista Team Blog.

The download is smaller through Windows Update -- 65 megabytes, compared with about 435 megabytes through the download center. (Microsoft explains that Windows Update uses "an efficient transfer mechanism to download only the actual bytes changed.") Microsoft says Windows Update is also set up to detect whether a computer has any device drivers known to experience problems after SP1 is installed.

Windows Vista users who aren't itching to download the service pack manually can make sure the automatic update option is turned on in Windows Update. The company says the service pack will start being distributed automatically to those users in mid-April, downloading in the background when the PC is online. After SP1 is downloaded in that way, Microsoft says, users will be prompted to install it.

What about getting an SP1 installation DVD in the mail? It might seem quaint in the age of broadband, but this has traditionally been a good option for dial-up users and people without ready Internet access. For now, they'll have to wait for it. A Microsoft representative says the company plans to make a Vista SP1 DVD available via traditional mail at some point, but it's not yet offering a specific date or other details.

What about stores? Packaged copies of Windows Vista with SP1 have been released to retailers, Microsoft says. The company says those copies will be sold in stores "as soon as retailers can make them available." Amazon.com is expected to start shipping the updated Windows Vista tomorrow.

When will Vista SP1 start showing up on new PCs? Based on what it's hearing from PC makers and retailers, Microsoft says April is its "best estimate for availability for PCs with SP1. However, there will be certain exceptions based on individual companies' distribution models."