When you want to install Forefront Client Security, you will receive this error message
"WUA API 2.0 is installed but the version of wuapi.dll is earlier than 18.104.22.1689"
This is due to older Windows Update agent version installed on your server. Proceed to http://technet.microsoft.com/en-us/library/bb932139.aspx to get the latest Windows Update agent.
Once you've install the latest version, you can proceed with FCS installation.