One way of doing it is with a KVM switch. My Belkin switch can take either USB or PS/2 as both input and output.

Probably overkill if you don't actually need a KVM switch though. I am not actually using my switch to switch video anymore. I hate the deterioration you get on video through the things and my new monitor has separate inputs so I switch with those now.
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Remind me to change my signature to something more interesting someday