I think you both can use ring: https://ring.cx/
Part of GNU and available under multiple plataforms :) also respects your privacy and your freedom it is so much better than skype.
You can install trisquel on a USB, dowload this: https://rufus.akeo.ie/ (is for windows) and use it to create Trisquel live usb.
Now the fact that you install Trisquel alongside Windows won't affect the computer at all neither windows but it is okay if you rather wait for your own laptop :)
