Is there a way to log bandwidth usage for Windows (XP) computers?
If possible, it should log this while remaining hidden.
Have a look at free, cross-platform BitMeter OS:
BitMeter OS is a free, open-source, bandwidth monitor that works on Windows, Linux and Mac OSX. BitMeter OS keeps track of how much you use your internet/network connection, and allows you to view this information either via a web browser, or by using the command line tools.
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Wireshark is a solid app for this, but not for the novice user. Another good option is Spiceworks as it is a little more user friendly, but best for domain environments. Another more convoluted method, but nonetheless effective for simple monitoring, could be to use SNMP on the PCs that you want to check bandwidth on.
o.O
How did you manage to interpret “log bandwidth usage” as a request for a packet sniffer?
Have a look at ntop. It would need to run locally but it can be run as a service so it would not be readily visible to the user and is accessed through a web browser.