I've never done it from Windows, but in various unix machines, I've done:

1. (perl was already installed)
2. Install Oracle client
3. Configure Oracle client (tnsnames.ora; set TWO_TASK, ORACLE_HOME, etc.)
4. Verify I can connect via sqlplus
5. Install DBI
6. 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.