Microsoft's latest build of Windows Vista Service Pack 1 has leaked from its invite-only list of testers onto the BitTorrent networks.

The 6.0.6001.16633 (longhorn.070803-1655) build weighs in at a relatively svelte 684MB, bloggers and message forum users claimed. The first Vista SP1 beta, which leaked earlier this month to file-sharing networks, was significantly larger - more than 3GB - and came as an .iso file that was, in effect, Vista with SP1 already added. The latest build, an .exe file, updates existing Vista installations.

Vista updates - such as security patches and other hot fixes - are normally packaged as .msu files, however, perhaps indicating that 6.0.6001.16633 is not a direct step toward a public release of SP1.

"Surprisingly, the update file is in the .exe format," said WinBeta. "Many (including myself) were expecting Microsoft to deliver the update for Vista SP1 in the new Vista/Server 2008-only .msu format used for current Hotfixes."

Odd-Magne Kristoffersen, a Norwegian blogger who specialises in Microsoft's software, posted a series of screenshots that he claimed show the newest build being installed. "The installation process involves no less (sic) than 3 restarts," said Kristoffersen.

Like the earlier SP1 beta, it appears Microsoft has been unsuccessful in keeping its testers in line; we easily found it on P2P networks.

Microsoft has been mostly mum about Vista SP1. As recently as a month ago, the company quashed rumours that that the beta was just around the corner. Its official line has been that it will issue SP1 beta "sometime this year." Many corporations, on the other hand, have taken the advice handed out by analysts, who recommended that companies delay deployment until SP1 is available.