To share a printer after the network is set up: via control panel on the computer the printer is attached to, go to the printer icon and right click on it, then properties, sharing, and click the button to share it.
When WinXP was new, you could easily share anything on your computer with other computers on your network. Since SP-2, sharing the root drive (C:\) is made more difficult, although you can override it. You can right click on any sub folder, or on a drive that doesn't have the system on it, and set sharing.
A very recent update removed the "sharing" that you get when you right-click on a printer. You can override there, but the "new preferred method" is to rerun the network setup on the computer that connects to the printer and specify sharing of printers as part of "registering" your computer and attached printer(s) on your network.
A problem with just right-clicking a folder and sharing it is that each individual folder that you click and share, and each folder that anyone else on your network clicks and shares, will then appear in Network Places on all the machines on the network, which can get pretty cluttered if you click very many of them.
Note that with a router, the router may have setup steps to handle all this, instead of doing it in WinXP on each computer.
Also note that when you first install a printer that you will share, the setup usually will ask if you want drivers to be loaded and shared. If you load the driver(s) "for sharing," any other computer that tries to hook up to your printer can get the driver from your machine at the first hookup.