Fake antivirus programs have become a favorite bad-guy scam for worming into your wallet, with a plethora of false reports of malware infections meant to to scare the unsuspecting into shelling out $50 for worthless software.
To help identify whether a pop-up or other warning is real, or whether it might have come from one of the many, many fake scams, Sunbelt Software yesterday put out a short guide. While it only lists a few relatively basic steps (check against a list of known bad apps, run a Google search, etc.), it's still good info to have.
Also, if you do happen to see a suspicious warning pop-up on your PC, it's important to know whether it came from a relatively harmless (though scam-oriented) Web site, or if it might signal a hidden malware infection on your PC. Bad guys often use an existing malware infection (like the Conficker worm) to surreptitiously install Rogue AV on PCs. Here's how to spot the difference.