Network Problem Please solve this.............

[Selvi]

As per our office setup we have 250 system with mixed of win2000 ,Linux and Solaris . also we have a router for our remote head office which is linked by 64kbps isdn leased line. we have not provided internet browsing in our network. so every one using leased line for internet browsing because the remote network has lot of proxy servers.   Now the problem is   Under normal browsing & downloading there is no problem with the link speed. If any one in our network starts download using Download manager to download mp3 or any such file the link gets very slow speed . so no one can work these times. These download managers takes whole bandwidth for one PC. so other peoples are affected by this. .  We couldn't find who consuming all bandwidth. Because it varies time to time also we don't have router enable password. In this condition   How can i stabilize our existing link? Is there any tool to find which ip consumes  large leased line bandwidth ? I have list of proxy server in our remote network . Is there any way to block all those address without modifying router configuration?   Your help will be appreciated   Thanks in advance   Balamurugan