Hi,
you can use the command
netstat -e on windows to get packet statistics which will return some lines of output. One line contains the data received and sent in bytes.
Write a script that executes netstat -e (shell_exec) two times, fetch the outpot each time and calculate the difference of the sent bytes.
After that you should have the used bandwidth in bytes.
You might need to measure the time between the two system calls using microtime and do another calculation to get the bandwidth in e.g. MB/s.
Since php 4.0.5 microtime should return correct values when used on Windows servers.
Thomas