1. You may have downloaded it knowingly or unknowingly mistaking it to be something else.
2. It may have automatically downloaded while you were visiting some harmful website
3. Your computer may have a trojan virus which in turn downloads such an application.

Recently I have noticed that the malware authors are also keeping track of the removal instructions published on the internet. As soon as they think that the malware is exposed, they change the file/folder names and the locations where they are saved on the hard disk. So keeping this in mind you will require the latest information of the malware in order to be able to remove it.

You can see the analysis report of

• Variation1) on this link
• Variation2) on this link 
• Variation3)  on this link

Trying system restore

 If you know the duration since your computer is infected, you can try to restore your computer at a prior date, that will work like a miracle in removing the infection

• Using system restore in windows XP
  •          Using system restore in windows Vista

Using free removal tools

• Special tools to remove a single virus or a family of virus.
• Free Online virus scanners
• Fully functioning antivirus/ antispyware

•  Free tools for windows XP
  
  
  • Free tools for windows Vista

Removing files manually

The steps to be taken to remove the infected files manually   

View Hidden Files

Before you could delete AntivirusPro and its associated files you need to search for them, and before doing that you need to enable to view hidden files and folders

•  View Hidden Files and Folders in Windows XP

•            View Hidden Files and Folders in Windows Vista

Boot in safe mode

If you are not able to delete the files and folders of this rogue application, in that case you should boot in safe mode and then try to delete it.

• How to boot in safe in windows XP

• How to boot in safe mode in windows Vista

Remove Processes from Task Manager

Optionally you can use Windows Defender to see the path of a currently running program/ process and its publisher, so as to differentiate malware processes from windows genuine processes.

• How to use Windows Defender in windows XP
  
  
  •         How to use Windows Defender in windows Vista

Removing entry from windows startup

  The system configuration can be started in xp and in vista by typing msconfig in the run box/ start menu search box. In xp by clicking on Start > run . The windows startup is reversible, therefore you can check / uncheck any entry from windows startup any number of times.

After the system configuration utility window is open, Click on the Startup tab, that will list all the programs that are scheduled to start when you turn your computer On. Expand the middle column using your mouse pointer so that you can see the path of the program on the hard disk, that will give you a clear idea, what program that is. Locate and uncheck the entries if  found "Antivirus Pro" (look for any suspicious name)  Press Apply , Press Close/Ok , at the next prompt select "Restart the computer"

 Unregister DLL

The first variation (antivirus_pro) saves a dll file in the location C:\Windows\system32\MSVolume.dll Therefore you need to unregister it first before deleting it from the hard disk. To do that, Click on Start > Run (in XP, whereas in Vista you can use the box that is already open) Now type the following, or copy paste the command below
regsvr32 /u C:\Windows\system32\MSVolume.dll
search on your hard disk, if the file name is MSVolumeAVP.dll, then use the following command
regsvr32 /u C:\Windows\system32\MSVolumeAVP.dll
Now you can safely delete it from the hard disk. 

Delete Folder

The first variation creates following folders
%CommonPrograms%\AntiVirus_Pro
%ProgramFiles%\AntiVirus_Pro

The second variation creates following folders
%Programs%\Antivirus Pro
%ProgramFiles%\Antivirus Pro
%Windir%\Antivirus Pro

delete these folders if found.

It is also found that it creates the following files in legitimate windows folders
C:\windows\system32\MSVolume.dll or C:\windows\system32\MSVolumeAVP.dll
delete the above files if found.

(in order to delete the files in the temp folder, it will be helpful to run a freeware temp files cleaner like CCleaner) 

%Windir% By default C:\Windows\ %System% is C:\Windows\System (Windows 95/98/Me), C:\Winnt\System32 (Windows NT/2000), or C:\Windows\System32 (Windows XP). %AppData% is C:\Documents and Settings\[UserName]\Application Data.
%CommonPrograms%  is C:\Documents and Settings\All Users\Start Menu\Programs (Windows NT/2000/XP). %ProgramFiles%  is C:\Program Files.
%Temp% is C:\Documents and Settings\[UserName]\Local Settings\Temp\ (Windows NT/2000/XP).

 Run CCleaner

Even  if you manage to find and delete Antivirus Pro  and its associated files , there will be leftover entries in the windows registry. If you run a free temp files/registry cleaner called CCleaner, that will help you to automatically clean the registry from the virus entries as well as the temp folder . Click here to read more

Edit registry 

If you are comfortable with regedit. You can edit the registry by using the windows built in registry editor. Click here to read more

More Problems

If you are unable to open Task Manager, registry editor, system restore, Folder Options etc

If the virus has disabled them. There are free tools and techniques to solve this problem.  They are listed here.

Tools for Windows XP

Tools for Windows Vista

Using the system file checker

If the windows is not functioning normally, you need to make sure if the genuine windows system files are not altered by the virus, you can use the system file checker utility to find it out and in case if they are altered then to repair them. See the appropriate link for your version of windows.

•  How to run System File checker utility in windows XP
  •          How to run System File checker utility in windows Vista

Unable to access security related sites

Using Firewall 

Check your firewall for any suspicious communication from your computer to the internet and block it using firewall.