Try cleaning your hosts file.
I spent about half a day on this, and none of these answers worked for me. I finally found the solution hinted at on OTN (the last place I look when I run into Oracle issues), and someone mentioned looking at the hosts file. I had recently modified the hosts file because this particular machine didn't have access to DNS.
I had a line for this host:
123.123.123.123 fully.qualified.domain.name.com hostname
Commenting out the line above allowed me to install the Oracle client.