MRTG is the indeed the one of the best solutions. the only problem is that it does need a significant configuration done by the person who install it. and it is limited to the HTML pages it produce.
You could use PRTG wich could be instaled direct to Windows without need to configure anything except the access to router or switch. PRTG as well gives the monitor for NETFLOW so you could nicely separate and have a look into different statistic data (eg which protocol are using the most bandwith, which PC have the most connections etc)
It is not a freeware but the do give a trial period so you could test it.
you can find it here
http://www.paessler.com/prtg/download 
