I marked Jim's reply as the answer as it will allow you to install Defender and he questions why you would want to do this.
I've also moved your post to the Install and Activate folder from Off Topic where I found it.
Basically, you should *not* be installing Defender as OneCare includes the protection of Defender and expands upon it by protecting against viruses and other malware. Of course, OneCare also provides other features not found in Defender - Firewall, backup, tune-up, etc.
So, the correct "further action" you should take is to run the OneCare cleanup tool Jim linked to and then reinstall OneCare and activate it with the LiveID you originally subscribed with and don't install Defender.
-steve