Well, while you might get some other opinions, Linux is a real operating system. It is properly designed to allow for different levels of security. While Win7 is getting better, the older versions often required users to be admins in order to run a program, such as the flight simulator. It works and it is much more stable that anything Windoze. You won't ever end up with a strange registry problem and it is much easier to contain the user's data to the home directory of the user, rather than being put all over the place. The main thing is that it works. For laughs, go look up the 49.7 day bug:
Notice how long it took for them to detect the problem and fix it, mainly because a Windoze machine is rarely up that long, whereas Unix/Linux based machines can keep running (literally) for years. This is the main thing that it can do that Windoze systems can't, stay up and running and more importantly working.
Linux is free with good support because many people who support it want to support it and don't like problems to not be fixed due to their egos, whereas companies often don't want to give you a free fix and would rather you buy an upgraded version.
In my opinion, if the person has no experience at all, it is easy to teach them how to use a Linux system, much easier than teaching them about all the spyware, malware, virus, etc. stuff that you need to know in order to protect a Windoze system. If they are familiar with a given platform, it can be more difficult.
A Linux system is better protected against users doing the wrong thing. While a user can mess things up for themselves, it is harder for them to do something to harm the system, especially if they don't have the permissions. Remember, Linux is designed to be a multi-user system.
The main negative issue is that not all programs run on a Linux system. You can use Wine or a Virtual Machine though.