The Intelliget Agent is installed by default, but you must start them on control panel.
This is a well known bug in metalink site.
On NT platform, you don't have to use dbsnmp_* functions.
These function are there only because the developers have forget to remove them. This work only on UNIX.
To start listener on NT, use control panel, start Intelligent Agent.
This work together with Data Gatherer. You have to start this service also. If Datagatherer don't start, check your registry, the oracle homes must be in order. e.g. if you have home1, home2, but don't have home0, you have to create a home0(dumy).

If your node have not discovered databases,
delete %orahome%/network/admin/snmp_ro.ora, snmp_wr.ora files, ./agent/*.q files and restart intelligent agent.

you can check for error on %orahome%/network/log/nmiconf.log file for errors, or check %orahome%/network/agent/services.ora for discovered services.