Mouse is double-clicking

I have an IntelliMouse Optical (USB) connected via a "data transfer switch"
(for using single set of kayboard, mouse and display with 2 PCs). The
connectors on this switch are PS/2, for the mouse I'm using an adapter
(suplied with the mouse).
I have installed IP5_2Eng.exe.

When I single-click on an object it is registered as a double-click, but not
alway, about every third or forth. It is annoying, really.

I've tried connecting the mouse directly on PS/2 of one of the PCs. Still
there. The switch is not a problem then.

Any idea?

Thank you.