In most corporate intranets, there is no way to get to the public internet. All you can do is web browsing through a proxy server.
That may be true in part, but nearly all users in a company can still access the internet. In most cases with Internet Explorer, going through a proxy server won't stop you from getting hijacked or having the entire machine compromised with malicious code. To really be effective, the proxy would have to do inspections at the packet level, rather than authentication, port filtering, agent/client checks, and URL based ACLs as most proxy machines do.