You can also set tags of your own in "Settings" tab of the devices like "win2008" for example. When in device list view (http://your.prtg.server/devices.htm) you can then use tags to filter and show only devices carrying a particular tag.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.