Can we install Windows on a Mac mini Can we install windows on Mac mini?

Also, is it possible to  Install Mac OS on PC's rather than Macs ?
Yes, Apple provides a Software (including the drivers) to install Windows a Macs: Bootcamp

Apple's EULA doesn't allow installing OS X on non Apple hardware. Although, some people have already done this (they call this Hackintosh) but it's tricky and you'll never have a fully functioning system (there are always bugs somewhere).