Something to bare in mind if you upgraded your SCOM 2007 environment to SCOM 2012 is that any AV exclusions you may have had in place for the monitoringhost.exe and similar processes will need to be redone. This is due to the installation path changing during the upgrade process.

The default installation path for SCOM 2012 RTM / SP1 is as follows:

Agent: C:\Program Files\System Center Operations Manager
Server: C:\Program Files\System Center 2012\