If you manually install PostgreSQL (with the installer from PostgreSQL: Welcome), don't use the Stack Builder at the end of the installation (which is a Tuning Wizard).
Either use the PostgreSQL Tuning option or -even better- just manually edit postgresql.config
Edit your PostgreSQL.conf file in the SQL 8.x/data folder.
Do not use the Tuning Wizard. On some systems it will make changes to the postgresql.conf it shouldn't, resulting in being unable to connect to the database!
If you experience this problem--> simply go to the 8.4/data folder, undo the changes that were made and reboot.
shared_buffers = MB (set this to 20% of your system memory, but not more than 1024 MB.... round it off to 128 MB, 256 MB, 512 MB or 1024 MB)
effective_cache_size = MB (set this to 2x the size of the shared_buffers)