Accessing computer remotely via website

Guys, I'm trying to find the best way to logon to someone elses computer via the web. They would go and access my site and somehow through the creation of a pin for security, I would be able to log onto their machine. Maybe they'd have to install an ActiveX controller but that's about it, I didn't want them to install a .exe file like teamviewer or citrix applications.

Dell use it to get into my PC. If you could poitn me into the right direction as to what I have to do in order to implement this (if at all possible), I'd greatly appreciate it.

Thanks
Yash