Did you read Assistant->SQL Database Drivers->QPSQL for PostgreSQL (Version 7.3 and Above) ?
How to Build the Plugin on Windows
Install the PostgreSQL developer libraries. Assuming that PostgreSQL was installed in
C:\psql, build the plugin as follows:

cd %QTDIR%\src\plugins\sqldrivers\psql
qmake -o Makefile "INCLUDEPATH+=C:\psql\include" "LIBS+=C:\psql\lib\ms\libpq.lib" psql.pro
nmake