@AdamB, this could be related to network congestion on the in-home WiFi network, or an issue involving insufficient bandwidth. Is the PC hard-wired or does it also use WiFi? -Kevin M. Cox Support Forum Moderator
The modem is a Cox Panoramic Gateway. The PC is hardwired, but even if it weren't, a single device connecting to the network shouldn't cause all other devices to drop their connection. Not for the amount I'm paying for Gigablast internet with "the best available modem."
A single device could drop a wireless connection. For example, a microwave oven could congest your ether with data-less radio waves. Wireless NICs read everything on your frequency so if your network gets overly bombarded, it could drop connections.
Every time your roommate boots a wired PC, your wireless connection drops? This sounds sneaky. Does your roommate's PC have a wireless card? It may be still enabled...and...have a higher priority than your devices.
A higher priority would be a QoS setting in the router. If your roommate configured your router to give the PC a higher priority than you, your router may have to renegotiate your share of the network. Log in and verify.
Who setup your router? I reads like either IP conflicts or some type of DNS Proxy setting. I'm curious if the other connecting-devices are configured for static addresses and you dynamic.
As Tiffany recommends, resetting to Factory Default would clear these "preferences" but I'd also find a follow-up procedure to better secure your network after resetting (changing default credentials, MAC filtering, Guest networks, broadcasting, etc). There are lots of articles to better secure your network...maybe even Cox has one.