I have used several different software to gain remote access to other's computers.

With a Unix system, it's easy. You use ssh and you can do everything you want (although some people just cannot handle a console, I'm fine with such, but ssh allows you to access X-Windows if you'd like with the -X command line option.)

With MS-Windows, since that system is just a desktop application, you need a special tool in order to achieve that feat. Most of them are for money or make use of things that do not work under Linux. Yet, there is one that I like, it is called TeamViewer. It was created by a German company and it works like a breeze. This is quite practical. You can find it and download the free commercial version from here:

http://www.teamviewer.com/

Although that's not working under Linux, it is still a really good tool. Quite easy and practical to use. Some other tools are complicated or make use of strange concepts / ideas.

Now, make sure you get the version from the main website. I once installed a repackaged version from "the wrong site" and ended up with all sorts of really unwanted things like toolbars for Internet Explorer (although luckily none of those were compatible with FireFox, my word!)