Sorry if I was a little unclear. You can certainly use another PC solely for the purpose of sharing a printer wirelessly, but it sounds like overkill. If I understand you correctly, you want to put your USB printer in the same room as your DSL modem / router anyway, right? Your modem also has wired network connections available too, right? As coffiejunkie mentioned, instead of using a PC, you can just buy a wired print server (~$30, and possibly even less), to plug the printer's USB port directly into one of your router's ethernet connection. The printer,even though wired, will be available on your network for any wireless laptop to access.
Regarding my other comments, an NAS is Network Accessed Storage, essentially a hard drive or drives in a box that connects to your network (usually wired, not wirelss), so that all of your networked PCs can share access to the drives. You can share files, stream audio or movies, even remotely access your files over the internet at other locations. Entry level NAS's aren't very expensive, and many of them have print servers already built in, so you plug the NAS into your router, plug the USB printer into the NAS, and both your USB printer and the disk drives are available over your network to all your laptops, etc.