The worm that’s infected millions of Windows PCs is a “very well-engineered” piece of malware. But researchers still have no clear idea what the hackers plan to do with the collection of computers they’ve compromised with “Downadup.”

Downadup, also called “Conficker,” has infected an estimated 6% of PCs worldwide . The worm spreads by exploiting a four-month-old vulnerability in Windows, by brute-force password attacks and by hitchhiking on USB devices like flash drives.

And effective. Most researchers, including those at Symantec, have said the worm is the most invasive seen in the last six years.

The faster hackers can come up with an exploit and put it on the street, the better luck they usually have, for fewer users patch their machines in the first days or weeks after a vulnerability is fixed.

Although some researchers now say that Downadup seems to have peaked — F-Secure Corp. Friday noted that its “growth…has been curbed” — researchers remained worried about the next step in the attack.

Most malware infects PCs so that hackers can then use the collected machines, dubbed a “botnet,” to send spam, attack Web sites or compromise more computers. To do that, the original attack code directs the now-controlled PC, a “bot” in security parlance, to download additional software.

But Downadup has yet to trigger such second-stage downloads.

The CNN-Microsoft project to create a commemorative 3D portrait of the presidential inauguration is live, and you can view it here. Using Microsoft’s Photosynth technology, the joint venture is designed to capture the historical event by stitching together digital photos shared by anyone who was there. The Photosynth software, part of Microsoft’s Virtual Earth software group, lets users create “synths”-3-dimensional renderings of multiple images of the same scene. Users can rotate and view the synths from many angles. The more photos supplied, the better the 3D effect.

The inauguration synth makes for an interesting view. The multiple vantage points do give the viewer a better sense of what the event must have been like live. Navigational arrows appear on screen, making it easy to move between photos, which Photosynth melds together, albeit not always seamlessly. You can rotate images as well. The effect is jarring at times, particularly if you’re prone to motion sickness, but the overall effect is pretty cool.

The real-time aspect of the inauguration synth is a lot of fun. A couple hours after the swearing-in, for instance, the 3D portrait had just a handful of long-range shots of the event. Within minutes, however, more photos were added, including several close-ups of Obama taking the oath of office. The CNN-Microsoft is an intriguing experiment in citizen journalism that may have a bright future.