Roommates using my internet, how do I stop them?

Ok I'll run Malwarebytes. I have used that before a looong time ago. Did not know it looked for key-loggers too.... As for knowledge the two guys who normally live downstairs are pretty much drug addicts one of them sits around and plays his XBox 360 and smokes pot during the day and not much else except tv. The other guy who normally lives downstairs just comes around once in a while to check on his cameras he has setup all over the outside of the house...lol They keep running out of power and need to be reset or something for memory as the other upstairs guy goes outside in front of them all day long for this or that...I think he knows the cameras are there and he's just messing with them that way...

Then recently a new guy who does not pay rent just moved into and lives in the living room downstairs during the night, then takes off with his computer during the day and sleeps , He is the one who knows something about computers and has talked (loudly) about what I am doing online with emails I send to Management... He has a good running knowledge of things apparently.

Can a person tell from the Netstat file(I'll upload it if you ask) I saved whether someone is connecting to my internet with another computer or modem(only one computer/modem is supposed to be hooked up, Mine) but they seem to connect all night whether mine is on or not. I am not being paranoid as I have heard them talking loudly about things I do on my internet account like the sending of emails when they should have absolutely no way of knowing about it.

I do not have a router since I only had the one modem and computer, will this completely negate their ability to access my modem so I could leave it on all night? I could setup the router with DNS or what ever its called so that only my computer would be able to connect to that but what if they have another modem their using? Can the other modem be setup so that it gives false info about itself pretending to be my modem instead?.. Sorry for all the questions but I would really like to get to the bottom of this and acquire the knowledge to protect myself from ID theft any more than has already been done...

I am the only person who can and does have a internet connection here at this residence according to Comcast. I pay for it monthly. The cable that comes from the outside of the house goes into a splitter for tv only services which no one else uses. The the cable must be going into the downstairs for the other rooms and common areas to have tv is someone wanted to have it.

This is not a shared service as I am the only person on the account, but the cable from outside to my room could be compromised it is to high up the two story house for me to see how it runs.

I am concerned about their sniffing my emails and personal information that I keep on my system (if they have accessed through the internet at some point in time) I need to run the above mentioned Malwarebytes program for that I suppose.

I do not have wireless at all, as I have turned off the wireless card in my laptop from bios/cmos and it does not even show up in the list of hardware. so no drivers are installed for it.. Hence My computer is hooked to the modem with cat5 and the modem is then connected to the wall with coax cable which then goes outside and is supposed to go directly to the outside wall setup by Comcast which then connects to the telephone pole. I think the guy who normally does not live there has simply accessed my computer through the internet. I can find no way to do this but I may be missing something. Any ideas what I can check? Maybe open ports?

See above for other info please. Thanks to all.
Robert

See below please I must have posted that after you posted a reply, Thanks! As for making sure of where my devices are connecting to how do I know? Is there a command prompt I can use besides netstat as mentioned?

Here's what is coming up with the ipconfig:

ipconfig
=====================================
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation. All rights reserved.

C:\Users\User>ipconfig

Windows IP Configuration


Ethernet adapter Local Area Connection:

Connection-specific DNS Suffix . : hsd1.wa.comcast.net.
IPv6 Address. . . . . . . . . . . : 2001:558:600a:8:300a:e3d7:ea3f:c119
Link-local IPv6 Address . . . . . :
IPv4 Address. . . . . . . . . . .
Subnet Mask . . . . . . . . . . . :
Default Gateway . . . . . . . . . :


Tunnel adapter isatap.hsd1.wa.comcast.net.:

Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . : hsd1.wa.comcast.net.

Tunnel adapter Teredo Tunneling Pseudo-Interface:

Connection-specific DNS Suffix . :
IPv6 Address. . . . . . . . . . . :
Link-local IPv6 Address . . . . . :
Default Gateway . . . . . . . . . :

Ok typed them into firefox and it gave a black screen with "Problem loading page" The server is taking to long to respond... What am I looking for here since I obviously got nothing of note?

Sorry, must have been a mix up because there is no router at all...lol Like I mentioned to one of the many replies above.

Also Malwarebytes shows nothing at all. I still think(but don't know) that it could be just a open port somehow or my AVG has been corrupted. Any thoughts?

Thanks Guys.

Well I sure don't own one, so it must be the guy downstairs that has my internet cable from the outside wall connected to his computer, then it goes to my room through his router and computer that's how he is seeing everything I am doing on my system.

What would you suggest I do for this problem how can I get enough data to provide the police with what they would need?

And thank you!
Robert

I do control the internet account like was said above please read everything here before making statements , ok?
They are stealing "MY Internet" I have MY name on the account. They have somehow hooked up an router in between the outside wall hookup and my room.

Yes they ARE Weirdos, and a whole lot more in that direction. but because I have no control over who moves into or out of this house I am stuck with dealing with them in the best way possible. First up on that list is to protect my system from invasion again, I do this by only leaving it connected when necessary to the modem. next I am attempting to get good help here and other sites like it(I do trust Tom's) How does my internet account not appear what it is?!? This statement is without thought... Don't bother answering here if you have nothing constructive to say...

I own the original modem and supposedly the only one hooked up to this account. According to Comcast This house cannot have another internet account setup on it because it is a house. At least that is what they tell me. I will be making contact with them again about this little problem of mine and see what they want to do since the use of my internet by others is considered internet theft of Comcast's internet not mine. They would prosecute the thieves so Comcast says. As for how I am hooked up please read some of the other statements above as I have already mentioned how. Thanks.