WIndows 2000 internet router at 100%

My network is Windows 2000 based (both servers and desktops). Whenever a desktop connects to the internet, no other desktop can connect to a site until the first connection is finished. When the first destop connects and begains a download (like Acrobat Reader), my router shows 100% and as a result, no other desktop can access the the internet. As soon as the first desktop has finished downloading, the router drops to zero and the next desktop can connect.This never happened with my Win 98 desktops. The internet connection has remained the same. Is that a tcp/ip fast start problem? is there a way to return to the performance that windows 98 gave me allowing multiple,simultaneous connections?