When you remove the ISAPI Filter plug-in from a Forefront TMG machine.
Steps
Log on with local administrator privileges and navigate to Start > Control Panel > Uninstall a program (under Programs).
Select Web Protection Solutions, then click Uninstall/Change.
This launches the uninstaller.
On the Remove Components screen, select Filtering Plug-in and any other components to be removed, and then click Next.
When the Stop Microsoft Firewall Service screen appears, stop the Microsoft Firewall service and then click Next.
Note: Leave the uninstaller running as you stop the Microsoft Firewall service, and then return to the uninstaller to continue.
Go to Start > Programs > Administrative Tools > Services or Server Manager > Tools > Services.
Right-click Microsoft Forefront TMG Firewall, then select Stop.
When the service has stopped, return to the installer and continue the uninstallation process.
Important: When the Firewall service is stopped, TMG goes into
lockdown mode. Depending on your network configuration, network traffic may be stopped. Typically, the Firewall service must be stopped for only a few
minutes.
When the following message appears, start the Firewall service and then click OK:
The ISAPI Filter has been unconfigured, you can now start the Microsoft Firewall Service.
Leave the uninstaller running as you start the Firewall service, and then return to the uninstaller to continue.
To start the Firewall service:
Go to Start > Programs > Administrative Tools > Services or Server Manager > Tools > Services.
Right-click Microsoft Forefront TMG Firewall, then select Start.
On the Software Removed screen, choose whether you want to restart now or later and then click Done.