The most secure PC operating system available today for private citizens is Linux or possibly UNIX. However, Linux is the much more popular product of the two so I will focus only on that operating system. The fact that Linux runs well on older, lower-powered PCs is a bonus. 

A few years ago, Linux had a reputation for being difficult to install, difficult to maintain, and also awkward to install new programs. Those days are now ancient history. The more popular Linux versions of today for desktop and laptop computers are easier to install, easier to maintain, easier to upgrade, and easier to add new programs than is Windows  or Macintosh. Anyone who is a computer novice should be able to learn to use a modern Linux distribution in a very short time, shorter than learning to use Windows. Most of today's more popular Linux versions are even easier to use than a Macintosh.

Comment: Watch the Mac users post messages disagreeing with me. Please don't post such comments unless you have used one of the modern, user-friendly versions of Linux for an extended period of time within the past 2 or 3 years.

On my desk, I have Macintosh, Linux, Android, and Chromebook, systems. I have quite a bit of experience with each of them

Also, Linux is cheap. Well, no... actually is is FREE. Linux-based desktop operating systems are just a free download away. Since today's Linux systems also run significantly faster on any PC than does the newer versions of Windows, you can keep using your older PC for several more years by using Linux. Another thought is that maybe you recently purchased a shiny new Windows machine and your old computer is gathering dust in the closet. If so, pull it out, download and install Linux, and learn something new. You may find that you like it!

If your present Windows  or Macintosh machine seems to be running slow, you might think about trying Linux. If you are frustrated by the constant updates and by all the privacy concerns of Windows, take Linux for a test drive.  In fact, you can even dual-boot your present computer if you still have enough disk space available. If you do that, when you boot the system, you will be asked if you want to run Windows or Linux. If you select "Windows," you will soon be running your present Windows operating system, complete with all your normal programs and data files in place.

If you select "Linux," you will soon be running the newly-installed Linux system. Not only will it be faster, it will also be much more secure. Viruses and other malware are so rare on Linux that you can essentially ignore the potential problems. To be sure, there are anti-virus programs available for Linux but I don't think many copies of those programs are sold.

Another option is to install Linux onto a CD-ROM disk, commonly called a "Live CD," or onto a flash drive. When you want to run Windows, boot up in the same manner you always have. When you want to run Linux, boot down, insert the Linux "Live CD" into the CD drive or insert the flash drive inti a USB connector, and boot up. You will then be running Linux without overwriting any information on your computer's hard drive (unless you deliberately want to overwrite something). Later, remove the Linux "Live CD” or flash drive, reboot, and you will return to normal Windows operation as if the computer has never seen Linux. 

The major drawback of "Live CDs" and flash drives is that running an operating system from a CD drive or flash drive is slower than doing the same thing from a hard drive. However, "Live CDs" are an excellent way of taking Linux for a "test drive;" try it out for a while before you make a commitment to put Linux onto your hard drive.

One of the major drawbacks is that there are very few genealogy programs available for Linux. However, there is one very good one and it is available free of charge. GRAMPS (Genealogical Research and Analysis Management Programming System) is free, open source, genealogy software. GRAMPS is easy to install, easy to use, and contains most of the features found in today’s leading Windows and Macintosh genealogy programs.

Of course, the most attractive part of GRAMPS is its price tag:  FREE. Versions for Windows and Macintosh are also available. GRAMPS may be found at https://gramps-project.org.

There are a few other genealogy programs available for Linux as well but most of them are a bit complex and require an understanding of Linux internals to make them work. For that reason, I do not recommend any of those programs to the Linux newcomer. The same is true for WINE, CrossOver, VirtualBox, and other products designed to run Windows programs on a Linux system. While all of them are very good products, the complexity of installing and configuring those products makes me hesitate to recommend them to Linux newcomers.

Want to install GRAMPS on your Linux system? If you can move and click the mouse, you can install GRAMPS and start using it within a very few minutes. That's true even if you do not know how to spell "Linux."

There are hundreds of other easy-to-use Linux applications that will perform the computer tasks that most people want: Firefox web browser, several powerful word processors, email programs, graphics and image processing programs, budget tracking tools, and lots of games. Most of these programs are available free of charge.

Of course, Linux can also use nearly all the web-based applications, including Ancestry.com, MyHeritage.com, FamilySearch.org, Findmypast.com, WeRelate.org, WikiTree.com, The Next Generation of Genealogy Sitebuilding, WebTrees, and many, many more cloud-based program as well.

Another "problem" (well, it isn't much of a problem) is deciding which version of Linux you want to use. There are dozens of Linux versions, called "distributions." Each has its own strengths and weaknesses. If you do not know which version is best for you, I suggest you start with Linux Mint. You can always switch to something else later, after you gain experience with Linux and can better decide which version best fits your needs. 

Comment: In this case, the word "Mint" refers to the tasty and aromatic herb, not to the place where they make money.

Linux Mint is a good place for newcomers to begin. It is available in two versions and both versions, the Mate and Cinnamon desktop environments, of Linux Mint should prove familiar to Windows converts. Since Linux Mint is based on Ubuntu (another popular and friendly version of Linux), there are a ton of compatible programs that are easily downloaded.

Another great thing about Linux Mint is that it only uses LTS versions of Ubuntu for its base.  Those buzzwords can be loosely translated as meaning, "this version of Linux is mature and is expected to be around for a long time to come."

Linux Mint's minimum system requirements are 2 GB of RAM, 20 GB of disk space, and a resolution of 1024 x 768. For a more comfortable experience, 4 GB of RAM is recommended. 

Recommended system requirements: 

That old PC in your closet probably meets those requirements.

You can learn more about Linux Mint at https://www.linuxmint.com/. If you would like to do some reading first before diving in, look at the documentation (available in many languages) at https://www.linuxmint.com/documentation.php.

So, is Linux and specifically Linux Mint perfect? No. Absolutely not. However, it is really good and will work just fine on most older PCs built 3 to perhaps 8 years ago. There is also a Macintosh version as well. Mac users will want to first do some reading by starting at: https://forums.linuxmint.com/viewtopic.php?t=321387.

Linux, and especially Linux Mint, is easy to install, easy to use, fun, educational, and available free of charge. What do you have to lose?