If your on a router this could be the problem.. some routers block connection to the local area network and there for you wont get a whois on yourself of browse either..
There is on thing to make sure of and its simple usually depending on your operating system.. you could make sure that there is a entry in the hosts file for localhost 127.0.0.1
For XP your hosts file is noramlly in C:\Windows\system32\drivers\etc folder open with notepad and just read through it to see. Its not unheard of with all the patches and hosts file changes that have been needed with WinMX for this to get lost somers.. If thats in there then its likely your router and some (very few) allow that to be changed in settings.. you could read thorugh your manual to find out about local connections and the settings.
If your like most folks it will be a router that is doing it and nothing you can do to fix it really ..