i would go into your router setup and make sure that you have the wireless connections set so that WPA encryption is turned on.
I would use a key from grc.com/passwords which is a pretty well random key you will get (hint: the more random the less likely someone could *EVER* get on your network)
If you don't have WPA and only have WEP turn that on (though this was designed to be secure a person can crack it in less then a minute because of faults)
(WPA has no faults that have been found so it would take YEARS to crack into it)
Over the past while I have noticed that my highspeed connection is slow at times. I am connected to a wireless router which is connected to my DSL modem. I have two PC's connected to the router by Cat 5 Ethernet cable and a wireless Laptop which connects wireless. I am wondering since my house is pretty closely build to the other houses beside me, is it possible that someone else could be on my network, connecting to the Internet through my router?