This proved to be quite easy. However, you cannot keep the database running.
- You first stop the database service from the Windows from the Computer Management console,
- Then move the data folder to a new location with more space,
- Then modify the path in the config file postmaster.opts, in the data folder itself,
- Then modify the path that's given to the service when it's launched by Windows : in the registry, edit the ImagePath string at key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\pgsql-8.3
- Then restart the database service from the Computer Management console.