I needed a SQLPlus environment recently to access the RAC DB I have running in a VMWare image.
I'd heard of the Oracle Instant Client, so I checked it out and man, it is dead simple. There's nothing quite like something just working as it is intended to.
To run SQLPlus as a client on Windows:
Download two zip files
* Instant Client Package - Basic Lite
* Instant Client Package - SQL*Plus
Once downloaded, extract them into the same directory -- which defaults to instantclient_10_2.
Then set PATH to include the directory.
And finally, set TNS_ADMIN to the directory where you have created and saved the tnsnames.ora file. I chucked my tnsnames.ora into the instantclient_10_2 directory so it was all contained.
Then it just works.