Question : How to Secure a Dlink wireless Modem from intruders ?

I have a DI-624    High-Speed 2.4GHz (802.11g) Wireless 108Mbps Router that my cable modem is connected to. I have 4 computers wirelessly connecting to this router and one is directly connected via a cable. My Internet conection was recently suspended that I had been uploading almost 2 Giga bytes a day. I did not have any P2P programs open nor have I uploaded anything in  the last month. When I called the Internet company they said that someone must be using my connection through my wireless router. My wireless network is protected with a network key and don’t know how someone can use my connection without accessing my wireless network.
 
I have several questions

1.      Is it possible that someone may be using my internet connection via the wireless modem ?
2.      If so how do I stop that ? Are there any firewall prorams out there that will help me make sure no one else accesses my router ?
3.      Is there an option in my router that will enable me to set up upload limits ?
4.      Is there an option in my router that will enable me to monitor network traffic with timestamps ?
5.      Are there any third party programs that will allow me to monitor the traffic through my router and maintain logs ?

Any help is greatly appreciated as I am getting frustrated because this is the third time my Internet connection got suspended for upstream abuse.

Thanks a lot ,

Sean.

Answer : How to Secure a Dlink wireless Modem from intruders ?

you have a wireless modem or a wireless router?  broadband modem - ethernet - wireless router - pc's
or both?  wireless modem - wireless router - wireless pc's

1) yes, change your ssid to one that is more unique - turn off ssid broadcast
2) enable encryption
3) no, but you can limit access from certain ip addr's or mac addr's
4) only, the router's log, which should show which mac addr connected to your router at what time & what ip addr was given to it
5) http://www.ethereal.com/         http://www.netstumbler.com/

for more info on dlink di-624 see here:  http://support.dlink.com/SupportFAQ/default.asp?model=DI%2D624%5FrevC