At a basic level, any PC's that are plugged into any consumer router are on the same network. Given that these routers (the cheap $30 or $40 ones you buy in stores) only usually have about 4 ports, you would need to 'add ports' by adding a bigger switch (8 or 16 ports) and have it plugged into the router (given that you have 10 PCs, assuming none of them are wireless). You have yourself a network.
A network usually implies sharing resources of some kind (files, printers, internet, etc). By default, routers will already share the internet connection if you have one plugged into it. If you want to share files/printers, you would need to run Windows XP/Vista's 'Network Setup Wizard' in order to share files, etc. With all the devices plugged in and running the wizard, you have a basic functional network where you can share files and printers (and the internet too).
