I think its quite normal one PC being able to max out your Internet connection, no matter how fast it might be.
In most cases, companies with fast Internet connection would place some type of throttling mechanism like Packeteer's solution smurf suggested, proxy server, Linux machine with class-based queues or a Cisco router that has various service policies configured to ensure everyone gets a chance downloading without maxing the line out.
The type of 'mechanism' you select, depends entirely on your network setup and budget. In the past, I've managed to successfully implement a 'cheap' Linux solution for large company here in Greece with a T1 leased line which had over 600 users behind it, and it worked like a charm once the fine-tunning was complete!
If your using a PIX firewall in your company, then depending on the NAT translations, you might be able to perform the throttling at your router in front of your PIX; But this is slightly more complicated as it would require no NAT translations from your PIX Firewall to take place, so the front router can see the actual internal IP addresses and throttle them;