Thanks for the response.
I understand that I'll only be able to use the peripherals, modem etc. on one machine at a time, but that's fine. All I need is a switch.
I'd like to be able to connect both PCs to the modem and other USB devices to save unplugging them and them in again each time I swap. It's not essential because they're not that difficult to unplug and they're not used as often as the KVM; it just seems like it'd be handy. If I can plug a USB mouse into a KVM switch, I don't see why I shouldn't be able to plug a USB hub into it, and then the mouse, modem and other USB devices into that. The KVM switch is mechanical rather than one with software and keyboard combinations.
On another thread (below), there was someone who seemed to have a device that switched KVM plus a few other USB devices. I haven't found that device for sale anywhere.
http://www.theanswerbank.co.uk/Technology/Ques tion734803.html
I specifically want a mechanical switching device, not one with software and keyboard combinations.