Help! Display issue with Mac Mini

I've got a very bizarre issue that has just cropped up with my 2.0 GHz Mac Mini, running a fully up to date Leopard 10.5.6.

The Mini is on a KVM switch with another machine and has been for over a year with no problems. When I switched back to it this afternoon though, the display had gone weird. It looked as though the whole screen was in negative.

Initially I suspected a problem with the KVM or perhaps the graphics card in the Mini. The other machine on the KVM still worked fine though. On rebooting, the Mini also seemed to be fine at first, but then as everything finished loading the display went back to 'negative' again.

I have an external with a couple of other boot partitions, one with an alternative Leopard 10.5.6 and another with my old Tiger 10.4.11. Both of those started up perfectly and had no issues with the display.
I'm presuming that means that there isn't a hardware fault!
Something must have got messed up on my main partition. Does anyone have any ideas? I don't want to resort to a re-install from my Time machine backup unless I have to, but I can't find anything in System Preferences that makes any difference.

Thanks in advance for any help...

 

Thanks Michael, that was it! I can't think how I could possibly have done that by accident, I must have lent on the bottom of the keyboard or something. I have tried to pick up as many keyboard shortcuts as I could since switching a few years back, but I wasn't aware of the universal access ones before...you live and you learn.