The secondary logon service is a standard windows service which is available on every windows computer. In the latest versions it is not started automatically. You need to start it via control panel > administrative tools > services. Starting it once should insure it works in the future also.
If that's running, SQL 8.4 should install. Did you try the new 8.4.5 installer from
PostgreSQL: The world's most advanced open source database ?
Both 8.3.12 and 9.0.1 are both fine, officially we don't support 9.0.1 yet (although it works just fine) but I'm using 9.0.1 myself now