First let's talk about this program. This malicious program has been around a few years, and as such changes name every year. This year, it's called, "Windows Antivirus 2009". Last year it was "Windows Antivirus 2008", and so on.
If the name wasn't bad enough, another problem is that this program disguises itself to look like a legitimate Microsoft program (which it isn't). The icon even looks like a windows icon.
You've heard of "adware" and "spyware" ... well this program can be termed, "malware", "rogue-ware", or "ransomware". In short, it's a malicious program that infects your computer and holds it hostage for money (Perfect Defender is another rogue program)
Folks tend to get it by surfing the web stumbling across a web site that is infected or responding to a scary popup message. Once this program is on your computer, it will bombard you with scary messages about many infections are on your computer and convince you need to buy the program to remove them. But sadly, even after paying your $50 for the program, the messages never go away - and you're still left with the annoying problem and out $50!
By the way, there's no hope of recovering your money from the folks behind Windows Antivirus .... but you can call your credit card company and tell them you're a victim of an internet scam and have them cancel the payment.
Good news is that there is hope! There is a program that remove Windows Antivirus 2009 (and other versions of it) from your computer. This program is called, Anti-Malware by Malwarebytes. I've used this program on some of my clients computers and it worked like a charm .... which is why I highly recommend others use this program!
Updated 28 September 2009: In addition to Windows Antivirus, this program also goes by the names, Alpha Antivirus, Perfect Defender, and a dozen or so other names. But all warning messages look and act pretty much the same way.