Hi,
some points

NTFS can allocate more then 2G;
your database runs in a NT-Service
see oradim ...
your dba-group is ORA_DBA
The sessions are always working as threads ( you see only one oralce.exe in taskmanager )
and a lot is to configure in the nt-registry like
TNS_ADMIN, ORACLE_HOME, ORACLE_SID ...

Orca