I recently set up a KVM switch (a Cables To Go TruLink with 2 ports) to share my keyboard, monitor, and mouse between a Linux system and a Windows 7 Home Premium system. My mouse and monitor work perfectly with it. Unfortunately, when I try to use my keyboard (a Microsoft Sidewinder X4) with the Windows system, no characters appear on my screen. However, I can still switch systems by pressing scroll lock twice. The issue is not with the keyboard as it works when it's connected directly to the Windows system.
Interestingly enough, when I switch to the command line Linux system, the keyboard works when going through the KVM switch.
Windows 7 system does check the connection status of all devices connected to it under "Device Stage" which was the new features/functions added to Win 7 OS. Most of the KVM switches currently in the market do not support or feed the continuously update of share devices (such as keyboard, mouse or even speakers connected to the KVM switches). That's the reason why
you only experience problem under Win 7 OS. The Windows 8 OS will have the same issue regarding the keep checking the connected devices status. It may be caused by both new OSs all have built-in support of touch screen display or under tablet mode.
So, the make sure your future KVM switch 100% support Windows 7's or Windows 8's"Device Stage", you need to looking for if they having advanced technologies, such as "USB DDM" or "Dynamic Device Mapping" for USB devices or "Full-time All DDC-EDID" for the display.
Hope this developers' insider knowledge-base can help you to under stand the reason(s) why your KVM work or not work.