Go go registry and first check for Actual Oracle RDBMS home NLS_LANG parameter entry under HKEY_LOCAL_MACHINE/software/oracle/Homex where homex is your rdbms home.

Do the same and check for D2K Home.

These two should match, If not change D2K NLS_LANG parameter. Be careful while playing with registry. If you are not sure. Make a copy of registr by exporting it to local drive and then modify.

For US, we use AMERICAN_AMERICA.WE8ISO8859P1
This is what on my desktop. I don't know what you guys use in Singapore. Whatever thats on your machine.