I'm bundling PostgreSQL as part of a desktop application installation, only to find out what was the recommended silent installation procedure under PostgreSQL 8.3 -- using the MSI file -- has been removed from PostgreSQL 8.4. Evidently, you're supposed to use command line utilities to effect silent installation now -- and it's not entirely documented. After some trial and error, I determined that the following sequence will silently install PostgreSQL 8.4:
1. mkdir C:\Program Files\PostgreSQL\8.4
2. net user postgres password /ADD
3. initdb --username=postgres C:\MyDB
4. pg_ctl register -D C:\MyDB
5. net start PostgreSQL
6. psql -U postgres -f MyDDL.sql