Oracle Database Discussions

I've oracle 11g installed on Windows Server 2003.
I have installed oracle 11g twice, so wasn't sure how to uninstall one version without effecting the other.
There are a few databases set up, none of which are now working.

Trying to connect using mysql client, or the oracle11g client results in TNSListener errors:
'TNSListener does not currently know of service requested in connect descriptor'

LSNRCTL> status
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=IPC)(KEY=EXTPROC1521)))
STATUS of the LISTENER
Alias LISTENER
Version TNSLSNR for 32-bit Windows: Version 11.1.0.6.0 - Produ
ction
Start Date 02-MAR-2009 11:54:42
Uptime 0 days 3 hr. 30 min. 1 sec
Trace Level off
Security ON: Local OS Authentication
SNMP OFF
Listener Parameter File c:\apps\oracle\product\11.1.0\db\network\admin\listene
r.ora
Listener Log File c:\apps\oracle\diag\tnslsnr\selene\listener\alert\log.
xml
Listening Endpoints Summary...
(DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(PIPENAME=\\.\pipe\EXTPROC1521ipc)))
(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=selene.compsci.cs.tcd.ie)(PORT=1521)
))
Services Summary...
Service "cs.compsci" has 1 instance(s).
Instance "cs", status READY, has 2 handler(s) for this service...
Service "cs_XPT.compsci" has 1 instance(s).
Instance "cs", status READY, has 2 handler(s) for this service...
The command completed successfully

Trying to modify the database (using Database Configuration Assistant) results in the error:
'there is an error in creating the following process: c:\apps\oracle\product\11.1.0\db\bin\sqlplus.exe -S /NOLOG the error is: Error 6 initializing SQL*Plus Messaging file sp1<lang>.msb not found SP2-0750: You may need to set ORACLE_HOME to your oracle software directory'

If there are forums you would recommend to a completely new oracle sysadmin please let me know.

Many thanks.