Operating system in a phone having no OS
If you can start your phone in fastboot, then use the fastboot utility to load your system.img and boot.img

You have a nice guide here: http://forum.xda-developers.com/showthread.php?t=2277112 but the main thing you need to execute is:

fastboot flash boot boot.img
fastboot erase system -w
fastboot flash system system.img
fastboot reboot


You will need a valid system.img and boot.img for your phone model.