it's possible the kvm switch isn't getting enough power. does it draw power from the keyboard ps/2 socket, or is it a plug-in version? my kvm switch is keyboard ps/2 powered (a 2-port kvm) and does not need the (optional) power adapter.
another issue people have with kvms is switching while booting. try plugging the "port one" into your desktop machine, and "port two" into the laptop. boot the desktop machine and let it fully boot before you switch over to the laptop. boot the laptop. can you switch back and forth now?
it's possible you're having problems due to unshielded kvm cables. the cables themselves should be
no longer than 6' due to signal strength issues, and should be of the
Shielded variety.
be sure that the puters are using the ps/2 (do kvms have usb options?) ports, and that they're not loose.
it could be the actual switch that's the problem, and you might want to try using it on another puter setup, or buying a new one, keeping the receipt for a return.
good luck, and if you continue to have problems, there's always Google
~Branson