Have you checked for proxy settings? Have you done any traceroutes to see if you can get to the server you're trying to goto? also try using telnet to make a connection to port 80 on the server you're trying to goto, this will help determine if a firewall is blocking it or if they have some kinda web content filter.
You also might try and dump the Administrator password has to your computer and upload it to plain-text.info and have it cracked, once you have it cracked then login as Administrator and see if you get more access. One word on this though, DONT take the easy route and just change the Admin password, this will get you fired faster than shit when they try and login to your box to do updates.
If that doesnt get you more info then the next thing you want to do is figure out what your domain controllers are and try and login with your admin password to those and see if that gets you more web access. If not then its no big deal, check through the hosts that are logged into the domain controller for any thing that looks like a proxy server or a content filter, the admin password will probably work on those as well so login and try and add your host to the "access to everything" rule.
If that doesnt work out for you then flip through the company directory and try and find a network admin, once you find a network admin go back to the domain controller and see what box he's logged in on, then login with your admin password to the machine he's on, disable the antivirus and throw a key logger on it. Once you steal his network passwords then check around on his box for some info to some kinda firewall box like a pix, then login to that and see if you can find any kinda rule sets that are limiting you from hitting the internet. If not then figure out what the local router is and setup your own route to bypass all the bullshit and go straight to the net.
If you're to lazy to do this then I can also recommend a great proxy script that will let you surf to one site then out to the internet through the script.
7-11 Cashier at spring valley and montford