I'll likely install the next Ubuntu LTS (due in May 2014) on my desktop. Normally I use the latest version of Linux Mint (am on it now), but there has been questions raised about it's security. There are many Linux based OS's built on the Ubuntu platform, some good, some not so good. Mint's usage actually shot past Ubuntu's, but still under the hood it's Ubuntu made over.
In addition to it's great security, Ubuntu (& it's variants) uses far less resources than any Windows install.
There's no such thing as a 100% secure OS, however most versions of Linux are more secure than Windows. The one thing to keep in mind, even if there's only one computer in the home, if one is connected to a cable/DSL modem, get a wireless router, install it & plug it into one of the usually 4 available ports. This gives the user a Hardware Firewall, as long as NAT is enabled. Usually it is enabled by default, but it's best to check anyway.
One can, but doesn't need to spend a fortune on a good wireless router for this purpose. Having a hardware Firewall is one of the best ways to secure any computer from intrusions. There are also tools in the package manager to scan for viruses/rootkits (ClamTK, chkrootkit & rkhunter).
As far as usability goes, if one can use the Firefox browser, that's the 1st step in learning to use the OS. Ubuntu has a massive community forum, with lots of professional assistance for free. Google is also your friend, it's highly likely that any issue that arises has been covered hundreds of times.
It also ships with a 100% Free Office suite in Libre Office. For many home/student users, it's all that's needed. Only a small amount of MS Office users needs all of the advanced options available.
Note that Ubuntu & many other Linux based OS's won't install on just any hardware today, PAE is required to install the OS or boot as a live DVD to give it a test run. It IS best to run it in Live mode to ensure it's compatible with the computer, plus it gives the user a feel for the OS. In this mode, one can also make transactions securely.
Best of all, this entire OS is 100% Free!
Cat