Remote IT similar to MS RDP

I need a client and server application developing that will allow up to 2 remote IT Support users to access some 30+ client PCs running XP, Windows 7 and Windows 8 platforms for remote IT support. Something similar to logmein or Thin VNC? The client PC should be a small Windows service that just sits there waiting for a connection. The remote IT support side can either be web-browser based or a custom .NET application. It should be able to connect via the internet and/or on a VPN connection on the same subnet as the office network using a specified TCP / UDP port. Once connected the remote IT support user would have full access to the users desktop like Microsoft RDP with the addition of a chat window. It should also warn the client PC user when connecting and ask if they want to accept the remote connection. Login security shall use either LDAP from a windows AD server or have a an encrypted usernames and passwords database or file.