Please note that the administrative privilege is not needed in the whole processes. MinGW would not add entries into register system or change the environment variables.
Download the automated GUI installer: mingw-get-setup.exe and run. Accept all the default settings and
Choose MinGW packages to install. Open MinGW Installation Manager installed in previous step, select Basic Setup in the left panel and choose package mingw32-base and mingw32-gcc-fortran. Then select menu Installation -> Apply Changes to start installation. The installer would download selected package and extract them into the given folder.
Go to the installation folder (C:\MinGW\bin) to check mingw32-make.exe and gfortran.exe.
Want to compile SWAT outside of Eclipse? Add folder C:\MinGW\bin to the PATH variable.