I wonder why?
I am using a Belkin switch too. One is between two Windows Vista 64 Bit PCs and one is between 1 Vista 64Bit and Win 7 64Bit.
I never needed to install any sort of software for it, on any of the machines.
To top it all, the Vista 64bit and the Win 7 64 bit are both on Dual Monitors.
I would suggest using a new mouse of a better make if possible.
"By God's Grace" it ought to do the trick....
I did forget to tell you that the Win 7 PC also dual Boots into SuSE and Windows 8 , all with the KVM and has never run into any trouble with any OSes so far.
One Last question, are you using a adapter to convert from the PS2 Keyboard/Mouse to connect to the Old PC? If so.... that's where the glitch is.
You need a better adapter or a new mobo with a PS2 Keyboard/Mouse on board connector.