If the 'McAfee Agent Backwards Compatibility Service' (macompatsvc.exe) is stopped or does not process requests reliably, the reason could be that untrusted injected .dll libraries have compromised the process and rendered it untrusted by McAfee Valication Trust.  Then, the VSE executable responsible for initiating McAfee Agent to update the DATs at the conclusion of installation (mcupdate.exe) might display an error message. That message displays in the user account session that is installing the product. If that session is the SYSTEM user's session (Session 0), the message displays on the desktop for Session 0. That desktop is accessible only to logged-on local administrators. Because the mcupdate.exe message might be displayed on a node that has no logged-on local administrator, it might appear that the installation has halted. The installation has actually completed almost all steps.