I've got a home network that access the internet using a router DLink DSL-500B.
Can I configure the Dynamic DNS service option in that router so I can directly update my OpenDNS account IP without client software on each different PC?
From a short google, I was not able to determine if your router actually supports DDNS, but if it does there should be a page in your router's config to set it up. For my router all i simply have to do is select the DDNS provider from a drop down, enter the URL, username, and password and it will keep it updated.
If it doesn't it support it, you shouldn't need to run the client on all of your computers, one should suffice (preferably one which is always on) since they are all on a LAN and have the same public IP anyways, and your IP should not be changing too often.