I have an old netgear router (cable) working as my wireless access point.
So I have a gigbit switch that takes care of the internal wired network, into which my second NIC on the core, MDs, telephone etc all plug into. The wireless access point (an old router in my case) is also plugged into the switch (on the router, make sure you use the port that the internet cable would normally plug into).
Assuming it is spare and you don't need the settings, I would recommend doing a factory reset on the router. Then, ensure that DHCP is enabled (have I got this right, anyone?), and away you go. You now have a basic WAP without security. Check it works, and then you can then set up WPA, MAC filtering, etc, in the normal way.
Hope this helps.