Check your DBSNMPW.LOG file for additional errors.

Try the following:
Remove the $ORACLE_HOME/network/admin/snmp_ro.ora & snmp_rw.ora files.
Remove all files in the $ORACLE_HOME/network/agent directory, leaving only the subdirectories.

Restart the Oracle Intelligent Agent.

Also, take a look on metalink at(search on Starting the Intelligent Agent):
Note:75063.1
Note 70115.1