I think someone is hacking my wifi? How can I tell and fix it?

I think someone is hacking my wifi. When I check my usage each day it is around 1.5 gig, even though I don't want videos and don't download from torrent sites. I have netflix, but rarely watch it. Yesterday was my test, I didn't do anything except surf the web and yet I am at 1.5gig down and getting close to the maximum. Any suggestions on what I should do to fix this?
I am using a Cisco Linksys router with a cisco valet for my wifi connection to my computer.
The other day, windows said "There is another IP attached to the network at the same time" and that was the first thing that made me think something was amiss. any ideas?

I do not know how to check if someone is using it but the first thing you should do is change your router password and your Wi-Fi password.

Jerry

Are they not the same thing?
I also have a newer router that I bought on sale a couple of years ago, it is a Linksys E1550, I was think about installing that instead, maybe my router is old (the firmware only states window xp, vista).
Also I do run spybot and super anti spyware regularly, but maybe that doesn't have much to do with it.

There is one password you use in your browser to get to the router settings, the other password would be from your computer to your router.

Jerry

Well interestingly enough, I cannot even get into my router password, the link doesn't work! I don't know if that is a sign of a hack job or just the router on the fritz, so I can't even change the password!!!

Go to the routers manufacturer page and look how to reset the router, there should be a button somewhere to push and hold and it will reset it back to the factory settings and then you should be able to get into it using the default password that comes with it.

Jerry