Great question, Tommy, and welcome to 7F. You can create your own Homegroup, and your roommate won't be able to access your files, period. You can think of a Homegroup as an exclusive club-- just because you're a member of one doesn't mean you're a member of another. Each Homegroup is separate.
Now, because you guys are sharing wireless (I assume) you will have a problem because Windows 7 will detect there is a homegroup already configured on the network. You will need to configure it for your wired connection instead. Try disabling the wireless connection first, setting up your Homegroup (which has its own unique password, btw, that your roommate won't know) then connecting the wireless again. See if that works, and let us know?