Alternative to Remote Desktop that doesn't interrupt user sessions

I'm looking for an alternative to Remote Desktop where I can connect to a remote computer and log in as any user without affecting any existing user sessions.

Basically, I need to access an application on a client's PC without interrupting the client's work on their PC. I know I can do this with Remote Desktop, but the additional configuration needed to accomplish this prevents me from using it (unless you guys have some tips on using RDP over the Internet with as little network configuration as possible).

Thanks in advance.

From serverfault Paperjam

• I believe any tool that allowed this on a desktop OS (xp,vista, 7) would be enabling you to violate the Windows License agreement.

  Paperjam : Good point. I was wondering why there were no Remote Desktop clones. I had not considered licensing.
  From Zoredache

• VNC? http://en.wikipedia.org/wiki/Virtual_Network_Computing

  Implementations by several vendors

  http://www.realvnc.com/

  http://www.tightvnc.com/

  http://www.uvnc.com/

  Farseeker : VNC does not create a new desktop, it interacts with the existing desktop on Windows, which is exactly what the op doesn't want.

  micmcg : So what the OP wants is more simultaneous connections? Thats controlled by the windows licensing then.
  From micmcg