2 or 3 pcs
one master, and 1 or 2 slaves.
the master have keyboard, mouse

slaves have or not keyboard and mouse but...
have PS/2 ports, com port or USB, or any else digital wire connected to any button on the slave machine (power on, reset, internal keyboard pins...)

then, the master will only launch an application when it wants to take the control of a slave.

briefly, the master will only generate binary signals that can be connected to any input on the slaves mother boards.

then, instead of using a software solution (with VNC for example), the use of a hardware solution will be more efficient. and the slaves will not need to run a software, and maybe, not need to be touched in order to start.

i plan to do something like this because my desk is not tall enough to contain as many keyboards and mices that i have PC's.

one pc to write, one pc to code, browse, one pc to play, then, it can be cool to share this idea, and maybe, make it real.

the first thing to do will be to connect a LPT pin to a common collector gate on the power switch or reset switch of a slave PC. the next step, try to generate usb mouse signals with lpt, and some external electronics.

there are exists a great thing: KVM (keyboard-video-mouse) switch. Use single set of keyboard, mouse and monitor to work with 2-4 computers easy (i use such at home). 8-ports KVM are very expensive, but 2-4 are not. also my 2-port switch switches sound also.

the problem with it is
netbooks don't have PS/2 port
old pc have AT connector for keyboard, and use COM port for mouse.

then, there is a need to unify this, the only way i think is possible is the use of external electronics, driver by a master pc, in order to generate signals for the slaves keyboard and mouse connectors.

Then it is better to make LAN and control slave PC via TCP/UDP. otherwise it might be better to look into MCU direction. also it is possible to use COM ports for connection (for netbook it is possible to use USB-COM converter, maybe can be taken ready as some cell-phone cable).

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how much old is your old PC? AT keyboard connector has the same signals as PS/2, there were adapters (now it is hard to buy them), but it's not hard to do it by yourself if you have ps/2 socket and DIN (?) connector. I made one of socket from dead motherboard and cable with DIN connector of dead keyboard.
same about mouse: your "old" board may have 5-pins connector. there were external optional ps/2 mouse connectors some years ago. you can buy it on "electronical garbage market", or made it similar of ps/2 socket and 5 pin-in-line "mom" connector (easy to get even now). there are four lines used, they differ from board to board, thus you have to find not connected pin first (it even may be absent), then find +5 pin and ground pin and connect them properly to the socket, left two pins are data and clock - connect them in any choice and check if mouse works, if not - swap data and clock wires and check again.
similar about USB - you may found USB as 2x4, 2x5 or 2x8 pins connector on the board. there are external optional USB sockets possible to connect to them, but you have ALWAYS check connectors for BOTH sockets for proper +5/GND pinning, as there are some different variants, misconnecting will destroy external USB device! if the board have USB, it is mostly may have USB keyboard support.
of course, ps/2 mouse, as well as on-board USB, has to be turn on in BIOS setup - old PC boards may have them turned off by default. also there are external USB cards, but ensure if it will support old PCI interface.

ok, then, i just need to output USB signals from the master, in order to give USB keyboard & mouse to the two others pcs.

then, i need to know exactlly what is send on this usb port (maybe it is time to make some oscilloscope with the sound card??).
usb port is binary, COM is binary, LPT is binary, then, it can be very possibe to do that without MCU, just a good program in the master PC, and some transistors to make the adapter.

make a lan is out. i want something really low level, the more as possible, but not the switch, because i really have 3 different interfaces for the 3 pcs about mouse and keyboard.
and i don't want to do it that way.

the master slave connection looks many mor interresting because it is external, and a simple execution of a .COM program can control the COM and LPT ports on the PC i want as master.

The USB port is very complex with initialisation protocols and special signalling requirements. To make a switch for it would be tricky. But if you still want to go ahead then the USB spec is fully open and available for download.

revolution speaks the truth! Between the USB port and the PC is a PCI device called the USB Host Controller connected to the USB root hub and driven by a very complex piece of software called the USB Host Controller Driver (UHCD). The UHCD maintains a Frame List which stores the Transactions defined on the USB interface. All transactions are defined by a set of complex packet-based protocols such as Control, Bulk, Isochronous etc. transactions. Yes, the specifications are all available - dozens of them, and many hundreds of pages!

Unfortunately there is no way to simply "bit-bash" the USB port like you could with the old RS232 serial port

if you can get up USB on the old PC, then you could use USB KVM switch to handle both netbook and old PC. otherwise you may spend a (half of) year inventing the keyboard and the mouse.