I recently had to resolve some TNS listener errors on a Oracle database server. It was Oracle 11g running on Microsoft Windows 2003 Server R2 SP2 32bit.
Basically the problem was that the memory Oracle was using was hitting the 2gb ram limit in the 32bit user space.
Adding the /3GB switch to the end of the command line and rebooting, gave Oracle another 1GB ram to work in and this resolved the problem.