As Nodsu says...
As Nod says the PS/2 - USB adaptor is not a standard
Now the reason you don't ket a keyboard error when using a KVM iis that the computer sees the KVM as a keyboard. If you installed the Keyboard drivers on both systems it would not matter you still cannot use the keyboard as; when using a KVM switch your keyboard is not talking to the computers it only talks to the KVM, the KVM is what in return tells the computer what to do eg. type "A" and so-on. You would have to install the driver on the KVM or get a USB KVM.
I know it is a hard thing to live with but that is life in the computer world, I have to go KVM on two systems and could not aford a USB KVM at the time so I needed to go with a difrent keyboard then the one I was using and realy liked, I was upset for a few days that I have to use the PS/2 as the USB is a $250.00 keyboard that I do need, so I put it one the system that I need it for and at this time I use it on my desktop aswell as a USB lazer mouse and the PS/2 is on my keyboard tray.