My PC had been running normally up until a couple of days ago when it suddenly started giving popup messages suggesting it's operating system was about to collapse because it was infected with trojans and spyware. A very official-looking Windows-type message then urged a full system scan. Clicking on this connected me to an apparently bogus spyware firm's website PC-Anti-Spyware which, you guessed it, offered to get rid of the trojans and spyware at a price. Then every 15 minutes PC Anti-Spyware operated as nagware with repeated popups that led to a new threat/offer: Get rid of what seem to be trojans and spyware and the annoying popups urging purchase, by making the payment. As the trojans seemed to be fake - my computer operated properly despite the claims of impending disaster - the main problem was the apparently unending stream of popup advertisements. Moreover, at $39-99US the anti-spyware software apparently does not even work.
My assumption is that this firm somehow installed or arranged to have installed these messages as a virus attached to a website I had accessed and then used the threat of them and the despicable popup sales messages to try to sell their bogus software. I say this because I have never contacted this firm nor sought any support or help from them in managing my software.
I searched the web for information and came to a website Spyhunter which warned of the PC-Anti-Spyware scam and offered to delete both PC-Anti-Spyware and the false messages. Again I was led through a series of webpages to yes, you guessed it again, a request for payment to actually remove the damaging programs. Again appalling stuff. I have no idea if Spyhunter is an offshoot of PC-Anti-Spyware.
I couldn't get rid of PC-Anti-Spyware using the McAfee package which I routinely run as background anti-virus software or by using old spyware removal favorites such as Lavasoft and Ad-Watch. Finally, I scouted the internet and found a number of favourable articles recommending Malwarebytes-Anti-Malware which I ran gratis. It seems after 24 trouble free hours (touch wood) to have finally got rid of the despicable PC-Anti-Spyware and the despicable popup messages although - being burnt several times in this last episode - I offer no guarantees regarding the use of this (or any) software.
This type of intrusion is socially costly. The cost to me was inconvenience over several days. Other people subject to the same scam would have eventually paid up and still had to endure the effects of the scam. I assume that legal action to prosecute these firms would be difficult to sustain because it might be impossible to prove that a virus had been loaded to provide the basis for a sale. Sometimes I despair of the ragged edges of free-wheeling capitalism and of anything-to-make-a-buck attitudes.