I've never done it from Windows, but in various unix machines, I've done:
- (perl was already installed)
- Install Oracle client
- Configure Oracle client (tnsnames.ora; set TWO_TASK, ORACLE_HOME, etc.)
- Verify I can connect via sqlplus
- Install DBI
- Install DBD::Oracle
If you're using ODBC, I'm not even sure that you need to have the Oracle Client installed -- and I've never used Instant Client, so can't give advice on that one.
See the 'Connecting to Oracle' section of the DBD::Oracle docs for more information -- I assume it'd be the same for Windows.