Oracle Database Discussions

I recently removed the 11g client (using the universal installer) and installed the complete 10g client on a Windows XP workstation. When I try to run SQLPLUS I get:

Error 6 Initializing SQL*Plus
Message file sp1<Lang>.msb not found
SP2-0750: You may need to set ORACLE_HOME to your Oracle software directory

I verified that sp1us.msb is in the C:\oracle\product\10.2.0\client_1\slqplus\mesg folder (size 12kb)
My path environment variable includes C:\oracle\product\10.2.0\client_1\bin

I tried setting ORACLE_HOME=C:\oracle\product\10.2.0\client_1
and I tried removing the ORACLE_HOME environment variable completely but in both cases I get the same error

I tried setting ORACLE_HOME=C:\oracle\product\10.2.0\client_1\ (note the trailing backslash) and I got a different error about Oracle not be functioning properly (sorry but I did not write down the exact error).

I have uninstalled and reinstalled the 10g client at least three times.

Help! What am I missing?