Computer screen 'mirroring'

[Big D]

Hi folks. I've got a friend who has a problem.
He badly needs a reliable way to 'mirror' the image on his monitor. I mean, turn everything on screen into a mirror image. This's just so that the monitor can form the basis of a home-made projector. One way to acheive this 'image inversion' was a physical reconfiguring of the wiring inside the monitor, but this proved to not be wholly reliable. I was wondering if any of you know of a programme that can do this same job? There's plenty of software which can 'rotate' the image though 90 or 180 degrees, but apparently nothing that can reverse the display entirely.
I'll deeply appreciate any help

Edit: I believe this is called 'yoke flipping', if that helps.