You could use the MAC filtering service to your advantage, and this means it's free. Setup MAC filtering for each PC on the network, and feed them into the router. Do it manually with the 'add device' command, or if choosing from a list, make sure you are not adding any stray PC's. Click apply or something similar, and then the filtering becomes active. To check for intruders, just go to MAC filtering, and click the refresh button embedded in the page, if there is not one, press F5.
This should show if any PC's are connected to your network without MAC authorization. They will appear under the 'available devices' tab, or something similar. Let me tell you that WEP is not too secure. You should use WPA-PSK or WPA or WPA2 if any are available. They are much more secure. Make sure you change the password for the router login page to something random, and alpha numeric. It's easy to hack WEP in under 5 minutes, and then you can access the router page and add your device's MAC to the list. Try and put these in place if possible, but if not, consult a security expert or IT technician.
