Connect WinXP PC to USB keyboard and mouse directly (without KVM switch), turn on the PC and check if the keyboard is OK?
If YES, go to step 02. If NOT, WinXP PC is not correct installed with USB keyboard, suggest to check BIOS setting. 02. Turn off WinXP PC, connect KVM switch (connect to the other port, not the previously used port), then power on with “passthrough mode” (that means switch on the port of WinXP and turn on WinXP PC), Please check if the keyboard is OK? If YES, go to step 03. If NOT, WinXP PC and KVM switch has compatibility problem.