Have you installed the drivers for it? On both computers? That is what is coming to my mind at first. Can you get into the BIOS? Pressing Del, F1, ESC during bootup? IF so then it is working properly, and you just need to get the drivers loaded into windows.
Not necessarly. I have a cheap small USB 2.0 hub that has an optional (but included) external power source. I recently got an iPod and when I tried to hook it up to that hub with the hubs's power supply plugged into the hub Windows recognized something was plugged into it, but it didn't know what. Then I plugged the iPod directly into my USB 2.0 card and it worked fine. So I don't think that just because it doesn't give you a no keyboard error doesn't mean that you can expect your keyboard to work.
How do you think a PS/2 keyboard works? It draws power from the PS/2 port.
Your problem is basically that the "USB to PS/2 thingy" is actually a workaround and not a real "standard" device.
You should get a PS/2 keyboard or a KVM switch that switches USB ports too.
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.