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Computerworld - Yesterday was a tough day for corporate IT administrators tied to McAfee. In some cases, they faced a full-blown meltdown of their organization's PCs, as hundreds, in some cases thousands, of Windows XP computers went down after receiving a faulty antivirus update from the security firm.

The whole story's not clear at this point, but there are some things that we know -- and a lot that we don't -- about the latest debacle from a vendor that is supposed to protect, not prang, PCs.

This is our first take on what happened, who was hit and why. If you have time to read this, we're assuming you're not one of those scrambling to bring crippled machines back to life.

What happened? Short answer: McAfee screwed up.

The long answer is more complicated. Wednesday's update -- McAfee pushes daily updates to its corporate customers -- was meant to detect and destroy a relatively minor threat, the "W32/wecorl.a" virus. Instead, it went rogue, wrongly fingered the critical "svchost.exe" file in Windows XP Service Pack 3 (SP3) as malware, and then quarantined it by removing it from its normal location. In some cases, the update actually deleted the file.

Think of the snafu as if the police pinned a crime on a suspect based on flawed DNA testing, only to find out they'd got the wrong guy.

Oops.

Why did the PCs crash and burn after getting the bad update? Without svchost.exe -- a generic host process for services that run from other Windows DLLs (dynamic link libraries) -- a Windows PC won't boot properly.

When users applied the update, then rebooted, they were toast: The machines crashed and rebooted repeatedly. Most also lost all network capability, and some were unable to "see" USB drives, a major problem since recovery may require the reinstallation of svchost.exe, something that could be done more easily by walking a flash drive from one crippled computer to the next.

What machines were affected? Only PCs running Windows XP Service Pack 3 (SP3), says McAfee.

Other version of Windows XP, including SP1 and SP2, were not nailed by the update, nor were systems running Windows 2000, Vista, Windows 7, Windows Server 2003 and Windows Server 2008. McAfee also said even older editions -- such as Windows 98 -- were unaffected.

There are, however, scattered reports on the McAfee support forum of Vista machines also going down.

My company runs Windows XP SP3. Why were only some crippled? Good question.

Only machines running VirusScan 8.7 were affected, users reported and McAfee confirmed. If you're running an older version, including the earlier Enterprise 8.5, you were in the clear.