Honestly I haven’t used Windows for years — you will find that even for compiling programs it takes twice or three times longer on Windows compared to Linux or Mac. If you are referring to the OpenSprinkler (microcontroller version) firmware, you just need to install Arduino and compile the program in Arduino. If you are referring to the Linux-based version (e.g. OSPi), I think Cygwin should probably do, but I don’t know how it handles HTTP server.
I actually would recommend you to install VirtualBox and then install a Linux virtual system in your Windows OS. That’s much better than having to use Cygwin.