If you want to do some development or hacking on the NI source code, I hightly recommend that you use CVS. This will make it much easier for you to track the changes you've made, port them into a new release or send me a patch. If you're new to CVS, read on!
For building NearInfinity with the existing Makefile, you'll at least have to have GNU Make and Find (you may also need Bash) and the Java 1.5 SDK. The simplest (IMO) way to get the GNU utilities under windows is to install Cygwin. CVS is also available under Cygwin.
To checkout the sources under CVS, use the commands:
cvs -d:pserver:anonymous@nearinfinity.cvs.sourceforge.net:/cvsroot/nearinfinity checkout -P NearInfinity/NearInfinity
No password is needed, just hit return (I can't test this right now, let me know how it goes).
You can make patches or look at changes using "cvs diff" (once you've checkedout the code with CVS, you shouldn't need the long "-d:pserver:..." option anymore).
cvs diff -u -p -w -b
cvs --help diff