I have a windows network (300 computers) and am just starting to use Splunk. I am interested in monitoring who is using the most bandwidth on the network. I am confused as to whether I can do that with just Splunk or do I have to have a 3rd party software (such as Solarwinds).
Install the Windows app for Splunk. It's free and just gets uploaded into Splunk.
It will probably not get you exactly what you want, it will get you most of the way there though. You will just have to modify the included Powershell scripts to capture exactly what you are looking for.