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.

More: http://pcworld.com/article/194855/