The printer will be no problem to share between systems, but the scanner will be a whole different story. You could do it with a Unix like operating system with a few simple symlinks, but Windows just isn't set up to deal with that sort of thing. The best you could do really, is remote control the computer with the scanner attached.
You could also use an A/B switch like you described assuming they make such a thing for USB devices, otherwise you're SOL with Windows.
I'm planning to buy a second computer (XP finally) which will find a place somewhere near my Windows 98 one. Is it possible to connect printer and scanner (both USB) to both computers, so that they can be used from both computers independently (not simultaneously, of course). There would be two computers to accomodate a family of four (well, there's a Windows 95 computer in the living room also) and printing is a rather common wish.
When printers were still connected via a parallel interface, there was such a thing as a data switch, with a big knob to be turned to 1, 2, 3 or 4 to connect 1 out of 4 computers to the printer. But that technology might be outdated.
I suppose everything goes quite well if I connect 2 computers, the printer and the scanner to a USB hub, as long as only one of the computers is turned on and the other turned off. But what would happen if both computers are turned on? Crashes, hardware damage, error messages, not being able to print?
I really wouldn't like to have to use two printers, nor to have to have two computers running to print from the one I'm working on. And I'm not yet ready to buy an Ethernet-capable inkjet-printer that can be connected to my router (if such exists).