Once the agent start, the Windows NT agent will first check the Windows NT registry values:
ORACLE_HOME and ORACLE_SID. The registry keys are located in:

HKEY_LOCAL_MACHINE => System => CurrentControlset => Services => OracleServicexx
The Agent checks the entire harddrive and builds a list of all ORACLE HOMEs.
This list also indicates the SID's in each ORACLE HOME. Another list is also
constructed for all the services in these ORACLE HOMEs (if more than one).
After all the information about SIDs, Listeners and other services are obtained
and analyzed, a summary will be written to the "services.ora" file. This file
is ALWAYS written to "ORACLE_HOME/network/agent" or "ORACLE_HOME/net80/agent"
directory. The "services.ora" file is used when the EM Console's Communication
Daemon or the Oracle Management Server checks for discovered services on a
specific node.

The Agent also generates two other configuration files "snmp_ro.ora" and
"snmp_rw.ora". These files are written to either "ORACLE_HOME/network/admin" or
"ORACLE_HOME\NET80\admin". The files can also be written to the "TNS_ADMIN"
location. The two files contain Agent configuration information. The
"snmp_ro.ora" file is read only and cannot be edited. It contains all the
services information the SIDs and ORACLE_HOMEs.