To add my two cents:
As others have said, it's much more likely a proxy issue. You can change your proxy settings, or download a browser (Firefox doesn't need any special registry settings, though your IT dept may not approve of it) and it won't have your proxy settings obviously.
'Corporate firewall' needs a bit of definition from you. Do you mean a site wide firewall? Then it can't be the firewall, since packets to localhost never leave the machine, never see the firewall.
Do you mean a firewall on your local box, installed by your IT dept? Then, in theory, they could set up a block to localhost. But then other apps may break, this is unlikely.
The old test used to be telnet. Telnet is the basest app, and never uses a proxy.
telnet localhost 80
will hit your webserver. If you get a timeout, then you have a problem.
If you get a response type in
GET / HTTP/1.0
And you should see your page. If this doesn't work, webserver has a problem, or you truly maybe have a local firewall. If it does work, you have a browser issue.
As far as other issues people mentioned, it has nothing to do with the hosts file. The hosts file just maps from name to ip address. There was a time pre-DNS and that's how all addresses were found. The special part is 127.0.0.1 which is defined by an RFC to be loopback and can not be on any network.
EDIT From looking at some of your other responses, you may not have had your webserver configured properly. You should verify you have a LISTEN port on port 80 with