On a Windows server, configured with Apache, adding the following line to httpd.conf to load libpq.dll can save you a lot of time :
LoadFile "C:/Program Files/PostgreSQL/8.4/bin/libpq.dll"
Note that you will have to change your folder accordingly to the installation path and version of PostgreSQL you have installed. Also note that having Apache and PostgreSQL on the same server for production environments is not recommended.
Cheers,
Serjik