So I go into my router config page, look under IP passthrough and see three computers using my router connection. One has my laptop's name, one has my PC's name, but I see a THIRD computer called "User Configured PC" I don't recognise at all. This user is always here when I look at IP passthrough, yet the two I own only appear when they're switched on. Should I be worried about "User Configured PC" or this just some setting desplayed by default?

I do use WPA2 with a long password, and ZA for firewall.Depending on your router this could actually be your router being displayed as a device on the network.

Can you check for MAC address filtering - don't actually switch it on, but there should be somewhere that lists currently connected DEVICES by their MAC address.

Double check the details against the sticker on the router's base.Do you have any other devices connecting through your (wireless?) router...such as a mobile phone, maybe? Quote

Depending on your router this could actually be your router being displayed as a device on the network.

Can you check for MAC address filtering - don't actually switch it on, but there should be somewhere that lists currently connected devices by their MAC address.

Double check the details against the sticker on the router's base.

Do you have any other devices connecting through your (wireless?) router...such as a mobile phone, maybe?