You should be using WPA or WPA2, Clive, which is far more secure, especially if the router is in close proximity to others.
You can use
https://www.grc.com/passwords.htm to generate a secure password (ASCII), which you copy into the router settings, and into the wireless settings on the PC. It should then be as secure as you can get it, but obviously, if it's not going to be used for some time, the best security is to power it off.