How do I use Microsoft Network Monitor to determine if my roommate is using P2P on our shared network?

I haven't heard of Microsoft Network Monitor. I use Wireshark - it can decode most common protocols. Sometimes if a service is running on an unusal port you have to override, e.g. HTTP running on port 7023 instead of 80, but generally it gets it right. Use a capture filter to just select your roommate's PC, once you have identified it on the network. Should be easy. You can also look in the raw network captures for ASCII strings. P2P is pretty obvious as there are connections to many end-user type addresses like 345.adsl.verizon.net etc. instead of the usual web stuff to akamai, yahoo, google etc.

Try and look at the port forwarding settings, look for things you aren't aware of. Also look at bandwidth. If you see a large spike in the amount of data downloaded and uploaded, they are using p2p You can also block their computer from accessing your network