How do you create a private home network?

We have a laptop running vista (wireless), a PC running XP (wired), and a Wii (wireless). I would like to set up a private home network (we have a modem with wireless built in) so that the laptop and the PC can share files and our printer. We already have all file sharing stuff (I turned on everything that had to do with sharing anything, not very safe :S) so that we can do this, but we would like to make it private just in-case. Also, we would like to be able to keep the Wii connected to the internet, as I'm not sure I'd be able to connect it to the modem again (very very difficult). If it helps, we are also using 802.11g and using WPA-PSK security. We can't change the security because then the Wii would lose connection, and because it's the only security type our modem has (other than 64 bit and 128 bit encryption). Could someone help please?