Use the built-in GDI screengrabber (no install needed) like this :
ffmpeg -f gdigrab -framerate 10 -i desktop [output]
This will capture ALL your displays as one big contiguous display.
If you want to limit to a region, and show the area being grabbed:
ffmpeg -f gdigrab -framerate ntsc -offset_x 10 -offset_y 20 -video_size 640x480 \
-show_region 1 -i desktop [output]
To grab the contents of the window named “Calculator”:
ffmpeg -f gdigrab -framerate 25 -i title=Calculator [output]
I found that framerate 10 suits screen capture well (you can change it).
I have encoded to both files and streaming outputs and it works quite well.