You need MinGW or M$ compiler (depending on the Qt version you have).

If you have one installed, open command prompt (there's a link in start menu called "Qt 4.2.x command prompt" or similar), go to the directory where the source code is, run "qmake" and then "make" (or "nmake"). If there is no .pro file with the sources, you will have to run "qmake -project" before "qmake".