For supporting end users' workstation machines remotely, here is the order you should try with the end users:
For server machines, we recommend using either Windows' built-in Remote Desktop (also knows as "Terminal Services") or a VNC-based tool. Remote Desktop provides each authenticated user a Windows logon session that is not shared. If your client lives in a place where the time zone is different, Remote Desktop should be your first choice as it doesn't need the client's interaction once Remote Desktop is enabled (typically it should have been enabled for a server for the ease for remote maintenance and monitoring). For servers, Remote Desktop is usually enabled via a group policy (AD GPO), although it can also be enabled through Windows System Properties.
Figure: Enabled Remote Desktop
Remote Desktop works for workstations, but it's not recommended due to a security risk if Remote Support isn't disabled. Also, because of the End User License Agreement (EULA), only allows 1 user at a time, if you logon to client's Windows machine, the client will be logged off.
If you can't use TeamViewer, Skype, or Remote Desktop, you can try VNC. There are a number of VNC servers and clients available. VNC-based sessions typically behave as if you're physically using the computer. This means that it shares the same logon session with the user who is currently logged on the machine. VNC software allows you to configure a specific username and password for remote access, which means that you don't have to share Windows usernames and passwords, or create a temporary Windows user account. Some clients may also prefer this as they can sit in and watch what is happening.
The VNC tools we prefer: TightVNC and UltraVNC.
Do you know the best way to handle a support call? - SSW Remote Support Standard
See useful tools The Best 3rd Party Network Tools - TeamViewer.