Remote viewers

I am looking for an application which would do the same sort of job as VNC without letting users know that I am going into their machines remotely. Ideally, the software would be open source or freeware. I need to be able to go into machines remotely and see what users are doing or not doing as the case may be. I am not keen on deploying any of the VNC packages as I see them as possible security risks.

Does anyone know of software which would do the above that I wouldn't have to pay for?