Microsoft Forefront Protection 2010 for Exchange Server (FPE) cannot coexist with any VSAPI-based antivirus product. If you previously used VSAPI in order to install another antivirus software product, you receive an error message when attempting to install FPE, and the installation fails.
When using VSAPI to install antivirus software, the following registry subkey is created to save information about the VSAPI library:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\VirusScan
If the VirusScan registry subkey is present in the registry when you try to install FPE, the installation will fail.
To delete the VirusScan registry subkey-
Click Start, click Run, type regedit, and then click OK.
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In the Registry Editor, expand the following registry subkey:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\VirusScan
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Right-click the VirusScan registry subkey, and then click Delete.
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Exit the Registry Editor.