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Computerworld - Security experts say it's the biggest worm attack in years, call it "amazing" and report that it infected nearly 9 million PCs in just two weeks.

Downadup is downright nasty. And that's even before it does much more than just spread.

But as analysts argue about how the compromised computers will be used -- to build a massive botnet, perhaps -- or how much information hackers will steal from infected machines, users like you have a more immediate concern: "How do I keep my PC from joining the ranks of the hacked?"

That's a simple question. Unfortunately, because of this worm's flexibility, the answers aren't.

What's the worm again? Thanks to the lack of an industry-wide labeling system, the worm goes by more than one name. Some companies dub it "Downadup," others call it "Conficker."

No matter the name, it's the same threat.

When did Downadup first appear? Security companies warned of the worm in late November 2008; Symantec Corp. was one of the first to sound the alarm when it raised its ThreatCon security alert level on Nov. 21. Within a week, Microsoft Corp. had added its voice to the chorus as it acknowledged a significant uptick in attacks.

However, the worm only really took off about a week ago as newer variations struck users and resulted in millions of infections.

How does it spread? One of Downadup's most intriguing aspects, say security researchers, is its multipronged attack strategy: It can spread three different ways.

The one that's gotten the most attention exploits a vulnerability in Windows that Microsoft patched nearly four months ago. The bug, which is in a file-sharing service that's included in all versions of the operating system, can be exploited remotely just by sending a malformed data packet to an unpatched PC.

But the worm can also spread by brute-force password attacks, and by copying itself to any removable USB-based devices such as flash drives and cameras. More on those two in a moment.

What machines are most vulnerable to Downadup attack? According to Microsoft, unpatched Windows 2000, Windows XP and Windows Server 2003 machines are at the greatest risk to exploits of the bug patched in October. That gibes with reports from security companies, which have highlighted the danger to PCs running Windows XP Service Pack 2 and XP SP3. Not coincidentally, those versions account for the bulk of Windows' market share.

Unpatched Windows Vista and Server 2008 systems, meanwhile, are less likely to fall victim to attack, since hackers must have authenticated access to the computer, or in other words, know the log-in username and password.