You receive an "Setup believes VSAPI based antivirus product is installed already" error message when you try to install Antigen in VSAPI mode, and the installation fails

When you try to install Antigen in Virus Scanning API (VSAPI) mode, you receive the following error message:After you receive this error message, the installation fails.

This issue may occur if you previously used VSAPI to install other antivirus software. When you use VSAPI to install antivirus software, the following registry subkey is created to save information about the VSAPI library: If this registry subkey is present in the registry when you try to install Antigen in either Extensible Storage Engine (ESE) mode or in VSAPI mode, the installation fails.

Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall your operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.
To resolve this issue, follow these steps:

1. Click Start, click Run, type regedit, and then click OK.
2. Locate and then click the following registry subkey:
   HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\VirusScan
3. Right-click the VirusScan registry entry, and then click Delete.
4. Quit Registry Editor.

If you install Antigen in VSAPI mode, you cannot run multiple anitvirus software solutions at the same time.