Antivirus System Pro is an irksome piece of malware that holds your computer hostage until you pay a ransom. It is nothing short of extortion. I just spent a few evenings removing it from a friend's computer, and the dozens of articles on the web (and even YouTube videos) were all wrong about how to get rid of this instance.
The program throws up all kinds of messages saying that the computer is infected, and occasionally sends Internet Explorer off to a porn or Viagra site for good measure -- I guess to make the "infection" seem more urgent. It also prevents new processes from starting, except for IE, since they presumably need to leave a way open for you to pay your ransom. The program sets up a proxy server for IE, though, so you can't get anywhere but Antivirus System Pro's site.
Usually in a situation like this, I use the Ultimate Boot CD for Windows. The scans I ran that way didn't find anything, though.
Other information on the internet suggests booting the system in Safe Mode and running Malwarebytes Anti-Malware to clean the system. This also didn't work.
There are several sites that give manual removal instructions. On my friend's system, none of the binaries or registry keys from the instructions existed.
Here's what I did that appears to have worked:
I also installed the newly free Microsoft Security Essentials on my friend's machine, since although there was a version of MacAfee on there, it was FUBAR. I'm not sure if that was something Antivirus System Pro had accomplished or if it being that way was what let the malware through in the first place.
All seems well for now...