try using this from Qt command prompt and make sure you are in the same directory of the .pro
file:
qmake -spec win32-msvc2008 -tp vc
if you are using VS2005, 2010, 2012, or 2013, replace win32-msvc2008
with win32-msvc2005
, win32-msvc2010
, win32-msvc2012
, or win32-msvc2013