There's a new threat to Windows computers that has been spreading like wildfire lately, and you need to be aware of how it attacks machines, what you can do to prevent becoming a victim, and what to do if you've already been hit.

The Downadup worm first showed up last fall, with the version that's causing the most trouble appearing around New Year's. Guesses on infections put the number in the millions. The worm sneaks into your system because of a flaw in the way Windows handles some network requests. You don't have to click on any downloads, or even visit sketchy websites to get infected; if you have an unpatched machine you can get hit just by being plugged in to the internet. The worm can also get into your PC by trying to guess your administration passwords or by copying itself to a USB drive that's then shared between computers.

Luckily, there's an easy way to avoid the attacks, and one that's been around for a while now. Last October, Microsoft released patches for all its current operating systems that plug the hole the worm uses to spread. Even Microsoft knew how serious the bug was at the time, making a special release for the patch, rather than wait to include it in one of its regularly scheduled updates.

The problem is that not enough people took the threat seriously and put off installing the patch, leaving their systems open to infection. If you don't configure Windows Updates to install automatically, you should definitely be making sure to check with the site often to see if any of these emergency patches get released.

Told-you-sos won't fix a compromised machine, but luckily Microsoft is on top of that one, too. It updates its Malicious Software Removal Tool once a month to help clean systems infected by these sorts of worms, and the latest version, updated this week, should take care of any Downadup infections on your systems.

What to take away from this: "critical" software updates really are just that, so don't dally with getting them installed.

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