Oops
These instructions are for building vips and nip2 in a cross-compiler from Linux. It's also possible to build the packages directly on Windows, but you're more on your own there.
First, you need to install MinGW. Ubuntu 10.10 includes a package for this that works for me. Older Ubuntus have broken mingw packages and you'll need to make your own.
Next, install jhbuild. This is a tool for automatically patching and building sets of software from source. Don't actually build anything with it, just do a basic install.
We have a set of scripts and config files on github:
git clone git://github.com/jcupitt/build-win32.git
Have a look at the README, set a few variables (you'll need to tell it where to find the cross-compiler, for example), and enter:
./unpack.sh
jhbuild --file=jhbuildrc --moduleset=vips.modules build nip2
./package-nip2.sh
and you should have your own nip2-7.x.x-setup.exe.
It can build just the libraries as well, have a look at the README.