fastboot does not recognize my LAVA Iris Atom
Right click on Not installed or Exclamation mark driver in device manager, click Properties, then click Details Tab. After that from Property Drop down list, click Hardware Id.
and copy first value from the box and google it by pasting in search.

i hope you'll get the right driver. 
Example:



Moreover, once you are sure that drivers are installed successfully, put your phone in Fast-boot mode to access otherwise fastboot command will not work.

in case you will stuck in process, let me know. 

Thank You