There's only one time when I have a definitive answer to the question "when should I upgrade my computer?" That's when the current computer is running a dead operating system.
Not just dead, but a Zombie!
Microsoft has a general policy of supporting their operating systems (XP, Vista, 7, etc) for 10 years after they're first released to the public. After that, they end support for those systems because they don't want to dedicate the resources necessary to keeping them up, and they want people to use their new software.
In the industry this is called a product's "End of Life" (or EOL). But when it happens, your OS doesn't just suddenly stop working, and nothing actually changes on your computer. But that's also the problem. Microsoft regularly pushes out updates to Windows to keep it secure and to fix bugs. After an EOL, Microsoft stops working on security patches for the OS. This means that while your computer may keep on working, it becomes less and less secure with each passing day. When hackers find holes in the operating system, those holes don't get fixed. That's when the computer becomes a zombie. And your zombie, like in the movies, can go around infecting others. Hackers will often use large networks of hacked computers (called "botnets") to carry out their nefarious work. Beyond this security concern, you'll start to notice that new software often won't even run on older operating systems because the developers don't want to put in the time.
So what does this all mean? Well, if you have a computer running either Windows XP or Vista, STOP USING IT! I can't stress this enough. It's time to move on. Chances are, it's a computer worth upgrading anyway. But XP lost support in 2014 and Vista saw its EOL in 2017. Both of these operating systems should be considered insecure. The bad news is that neither one can be upgraded to a newer operating system without completely wiping the hard drive and starting over, so a new computer is the sensible way to go. Feel free to call for suggestions on new systems!
Fortunately it's a little early to worry about the EOL for other operating systems. The incredibly popular Windows 7 doesn't lose support until January 2020. Don't quote me on this, but I wouldn't be surprised to see that support extended a little due to 7's popularity - XP got two extra years after all - but nothing is for sure and I'd treat 2020 as the deadline. Windows 8 is supported all the way until 2023!
Windows 10 will be tricky, as Microsoft currently plans to make Windows 10 an ever-updating operating system, so theoretically after 2023 there won't be any new zombie operating systems.
As usual, if you have any questions please email me!