Windows servers are accessible via Remote desktop sessions, remote desktop exists on most Windows PC's and you can usually start it by pressing the Windows key then type remote in the search box. Server IP, default user ("Administrator") and default unique password can be found on your dashboard at Dashboard -> My servers. Linux clients may install the rdesktop package in order to access a Windows servers via remote desktop.
Another option is to connect directly via the KVM VNC console found at: Dashboard -> My servers in the servers drop-down menus. Press Connect to connect with noVNC from a new browser window (make sure the popup blocker is disabled for control panel). Should you accidentally lock yourself out with a bad firewall rule you can still access your server using this tool.