Antivirus System Pro attack

I am being attacked very persistently by Antivirus System Pro. It's giving me an "Antivirus System Pro alert", a "Windows Security alert" and a "Spyware Alert" with "34 serious threats". My system is also bringing up "porno.org", "porno.com" and a Viagra ad that I did not request and in which I have no interest. As stated, it is very persistent as each time I delete an alert it simply pops up again within 20 seconds. How do I get rid of it?

Please download the current version of HijackThis from HERE

• Double click and run the installer.
  
• It will install to C:\Program Files\Trend Micro\HijackThis\hijackthis.exe
  
• After installing, you should get the user agreement, press accept and Hijack This will run.
  
• Select Do a system scan and save a log file. This will open a notepad file of everything Hijack This found, copy and paste it back here.