After months of silence, Microsoft finally coughed up details Tuesday about its plans for the first update to Windows Vista, saying the service pack will arrive in the first quarter of next year.

In the next few weeks, Microsoft will start private testing of a beta of Service Pack 1 for Vista as well as a third service pack for Windows XP. The company plans initially to release the beta only to 10,000 pre-selected testers, though it may expand that release later. A small group of testers are already working with a “beta preview” version.

As for what’s in the Vista update, it’s mostly a collection of existing fixes and tweaks aimed at improving stability and reliability of the operating system, which went on sale to consumers in January.

“It is not a delivery vehicle for lots of features,” said Shanen Boettcher, a general manager in the Windows unit.

There are a few minor enhancements, most notably the ability to encrypt multiple hard drive partitions using Vista’s BitLocker feature.

“Lots of folks gave feedback that ‘We have an OS partition and a data partition and we’d like to encrypt both,’” Boettcher said.

Also being added are support for an emerging removable storage file format known as exFAT as well as for EFI (extensible firmware interface), an alternative to the BIOS (basic input output system) that handles the initial start-up of a system.

Vista SP1 will be a large download: Roughly 1GB, based on current test versions. By way of comparison, Windows XP–the whole thing–shipped on a CD, which only holds about three quarters of a gigabyte. Installing the OS upgrade will require 7GB of free hard drive space, though much of that will be returned to the user once the megapatch is applied, Microsoft said.

The key question is what, if any, impact the contents of the update or its timing will have on the plans of large businesses to move to Vista. IDC analyst Al Gillen predicted that it won’t have that big of an effect.

“It doesn’t fundamentally change the landscape for Windows Vista adoption,” Gillen said. Windows Vista service pack 1 (SP1) via [cnet]