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