Windows – 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.