I am sharing internet connection with five of my housemates. However, I can't see any others computers in My Network Places but my housemates can see mine from theirs. I wonder perhaps it is because I turned off some sharing functions, I derno. If i did, I have no ideas how to restore it. Any suggestions?

Also make sure you've enabled File and Printer Sharing for Microsoft Networks on network card's properties, if not you won't be able to share file or printer. Open a browser and type \\<your-housemate-PC-ip>, then hit Enter, see whether you can access them. You can also ping your housemate IP, if you can't ping then there is firewall enabled on their PC to block your ping (most likely). Can disable the firewall to test again. If need to use firewall, then the file sharing traffic and other required application traffic need to be allowed on firewall. Hope helps..