I don't see why not.
You will need to switch on either WEP or WPA encryption on in the router settings. WPA is more secure.
Then go into the wireless settings on each PC and try to connect to the network. You should get some sort of option saying that the wireless requires a password to connect. Type in the password that you set in the Router settings and you should have a connection.
Once you have the connection sorted you could chose to turn off the SSID (Network Name) so that the network will be hidden from other people.
Also, don't forget to put a password on your Router settings access!
Another thing you can do is specify IP addresses to each PC (ie 192.168.1.2, 192.168.13 etc) then in the Router settings only allow connections from computers with the IP addresses you set. This will only cause a problem if you bring in a new PC to connect to the network as you'll need to adjust these settings to allow more IP addresses.