Okay, thanks, it isn't a DOA problem. It's an Oracle client problem.

Our network admin installed the client from 11.2.0.3 onto Windows 7 when he built the installation image. Although the OCI.dll properties read 11.2.0.1, it isn't backwards compatible when reading from an 11.2 database version before 11.2.0.3.

The search that I ran to eventually find this out was:
problems with Oracle client 11.2.0.1 and Windows 7 and expired password
which includes gems such as;
http://feed.askmaclean.com/archives/11-2-0-3-client-password-expiry-and-ora-1017.html