Been doing a lot of rails work on windows recently and ran into some issues that needed solved. Just like on Unix, to get the RMagick gem working on a windows platform you need to install ImageMagick. Go to http://www.imagemagick.org/script/binary-releases.php?ImageMagick=uteqfietdab37ghqrvfuvoul55#windows and download http://www.imagemagick.org/download/binaries/ImageMagick-6.7.3-7-Q16-windows-dll.exe
Double click and go through the installer, selecting to install C and C++ header files and libraries from the additional options page (this is very important)
Then add environmant variable CPATH=\path\to\imagemagick\include and LIBRARY_PATH=\path\to\imagemagick\lib
You can then ‘gem install rmagick’ and it will build it using the native image magick libraries.