I had a similar problem where a user had installed an old (demo ?) version of McAfee Firewall 8.x (or was it VSE 8.x) and then we installed our agent and HIPS and VSE. The HIPS install failed and the FW8.x didn't get uninstalled.
In the end I had to (manually )null disable and uninstall all the products except the agent, including cleaning up the registry. After that the agent was able to download and install everything correctly.