your roomates' and your PC may have possessed similar IPs thus creating an conflict error.. a similar pop-up message should also appear in your roomate's PC.

to resolve it, you got to have access on both PCs to individually set their IPs to different values..
you can either assign them manually or could just let the router assign them automatically.. but since you stated you have little knowledge about networking, i would suggest you do the second one..

to do so, you have to perform the following steps separately on both PCs...

• open Network Connections from either Windows Explorer or Control Panel
• right-click on the network device adapter icon, and select Properties
• from the Properties window, scroll down on a menu list found under the General tab and find the Internet Protocol (TCP/IP) item
• once found, click to highlight it and then click the Properties button just below it
• under the Internet Protocol (TCP/IP) Properties box, enable "Obtain an IP address automatically" and "Obtain DNS server address automatically"
• click OK to apply and close all opened windows and dialog boxes.. after this, the router should automatically assign unique addresses to each PC in the network, and the IP conflict error should never bother any of you anymore