Monitoring network for small business

We are looking for monitoring the network in our small business(43 users) including 3 servers and know where the bandwidth is being used. In addition, we need to know the curve of the internet usage per usage over the time and check if there is any virus that is using the bandwidth and on which PC which is shuting down the network. We need to have a trial of the software showing the curves. In addition we are facing some issues with a monitoring software using snmp which is showing communication lost as we need to know why the communication is lost and where the packets are being dropped for a particular equipment