If you install Forefront client security on any server in your MOSS2007 farm you might find that things suddenly stop working and you get ISAPI filter loading errors appearing in your event logs.
Here’s how to get up and running again.
1) Read this article. kb952167 - Folders may have to be excluded from antivirus scanning when you use a file-level antivirus program in Windows SharePoint Services 3.0 or in SharePoint Server 2007.
I would suggest implementing all of the folder exclusions mentioned in the article to all servers in your farm.
2) Ensure that the IIS_WPG and IIS_ADMIN_WPG groups both have Read and Execute permissions to the following folder: C:\Program Files (x86)\Microsoft Forefront Security\SharePoint.
You will need to set this permission on each server in your farm.
You should be good to go.