Facebook told QueenPendragon that her PC was infected. The message looked very much like rogue malware trying to trick her into downloading something vicious. She asked the Antivirus & Security Software forum for advice.

I have to admit that when I first read QueenPendragon's story, I assumed she caught a rogue.

I was wrong. The message actually came from Facebook.

You'll be happy to know that Facebook doesn't intentionally infect your PC, or scan your hard drive looking for malware. (Why should they? They already know so much about you.) But Facebook does look for suspicious behavior in how you access the social networking site. For instance, if you appear to be sending out a great many messages in a short amount of time, there's reason to believe that a botnet is sending them out through your computer.

In those situations, Facebook may block your access to protect other users, and recommend you scan your hard drive.

The company has a legitimate concern. Remember the Koobface Trojan? You'd get a message through Facebook, appearing to be from a friend, trying to get you to play a particular video. Only the video wouldn't play until you updated Flash. And that "Flash Update" turned your PC into a botnet-controlled zombie. Obviously, it's in Facebook's interest to keep that from happening.

Facebook's solution to a perceived threat is for you to scan your computer with a special online scanner that MacAfee makes available specifically through Facebook. I tried the MacAfee scanner out, and found it harmless, in that it doesn't load a lot of junk onto your PC (although it may require you to update Flash).

You might want to augment it with a scan by the security software of your choice (when you have reason to be suspicious, the more scans the better), but you'll have to run MacAfee's to get clearance from Facebook and have everything working again.

Read the original forum discussion.

Contributing Editor Lincoln Spector writes about technology and cinema. Email your tech questions to him at [email protected], or post them to a community of helpful folks on the PCW Answer Line forum. Follow Lincoln on Twitter.