I've used the VNC protocol successfully for this. VNC chops up the screen into regions of images to be sent. So the shared computer must have fast upload speeds.
RealVNC is a fine implementation from a company of the original VNC team. Client is free of cost, the server is inexpensive. Runs on multiple platforms.
FYI for other readers, Apple includes both a VNC server and client with Mac OS X, called Screen Sharing.
Skype has built-in screen sharing. Has worked for me for business meetings and demos.
Firefox is one of the first web browsers to support a new spec for audio/video chat built right into the browser with no plugins. The Firefox implementation is called Hello. This page says it has some screen sharing ability but may not be enough.
In a pinch I have simply asked the user to use their video camera on their phone to shoot the screen while they work the computer and talk to me. I talk them through the issue correcting them as they go. The cameras are so good now this is surprisingly effective.
Specifically I've used iPhones with their bundled FaceTime app