It is very important to keep computers up to date with security fixes. Worms and viruses can use a lot of network bandwidth, and are normally prevented or fixed by applying the appropriate operating system patches.
Windows machines in particular require many patches, and downloading them also uses much bandwidth. This can be reduced by centralising updates with the Windows Server Update Services. Usually a Windows machine is left running 24 hours a day and regularly checked for available updates, which must be selected to be deployed to the other machines on the network.
Antivirus software is not essential to bandwidth management, although it can help to control network worms. It may also use significant bandwidth to download updates. Most good antivirus software has systems for corporate use, designed to download updates automatically onto one computer and deploy them across the network, but they are often not easy to use, and sometimes expensive.
Traffic shaping can help to ensure that large downloads don't interfere with the speed of Web access, by restricting the bandwidth available for the downloads, and prioritising web traffic over downloads. The Linux and BSD open source operating systems have such features built in.
For more details please see the Traffic Shaping page.
Universities and other shared networks often allocate a quota of bandwidth to each user, on a daily, weekly or monthly basis. This helps to reduce excessive usage, such as constant high usage, while still allowing fast network performance for occasional users. For example: