Currently, Csound 5 builds and runs on Windows using the MinGW (http://www.mingw.org) environment with the MSys shell (http://www.mingw.org/msys.shtml). This environment is free, open source, and emulates the standard Unix/Linux environment and tools. On Linux, Csound 5 builds using the standard tools. Unix should work the same way as Linux.
If you have properly installed all the dependencies mentioned above, you can build Csound 5 and CsoundVST simply by opening a console, changing to the csound5 directory, and executing the scons command. To see the various configuration options, execute scons -h.
Once you have built Csound 5, you should run scons install which will copy the Csound 5 files (and CsoundVST files, if you have built CsoundVST) to the appropriate directories on your system. Also, if you used any commandline flags when building Csound, you should reuse those flags when running the install target. For example, if you built using scons gcc3opt=pentium4, you should install using scons gcc3opt=pentium4 install.
You must use a version of scons no later than 0.96.1.
It is highly recommend that you update your MinGW installation from the SourceForge site to the "current" level for core gcc, g++, binutils, utils, and the Windows API headers and libraries (w32api).
Rebuild and install a version of libsndfile no earlier than http://www.mega-nerd.com/tmp/libsndfile-1.0.10pre4.tar.gz.
PortAudio works with either the Windows multimedia libaries (./configure ---with-winapi=wmme) or with ASIO (./configure ---with-winapi=asio). At this time, low latency on Windows is only feasible with ASIO, but it is not as robust as the multimedia library.
The MSys shell does not allow the user to execute Python commands directly. Therefore, you need to make sure that Python is in your Windows executable path, and run the build like this:
$$ python c:/tools/python23/scripts/scons
You may also need to customize the custom.py file to declare to scons the locations of required header files and libraries, since on Windows there is no standard location for these as there is on Unix and Linux. You may not need to modify custom.py if you install all third-party libraries in the MSys /usr/local/include and /usr/local/lib directories.