Computers change very quickly. They double in performance about every 18 months, and continually gather new abilities. Software moves along quickly too, so by the end of year three, brand-new software expects to run on machines four times the speed. No matter whether it's ace or awful, in three years your machine will struggle to run modern software. Some PC tasks are less speed-sensitive than others, of course.
Office software is generally quite undemanding.
Games software, on the other hand, often pushes the envelope in the biggest way, while processes like video editing and CD writing go a lot more smoothly if the base machine has bits to burn. So if you want to run any software available on the retail shelf, you need a PC made relatively recently. This has been a fact of PC life since before the dawn of the age of Windows. The hardware will run for a lot longer than three years, of course. Given reasonable care, most standard machines survive five or six years without problems, while some will make it to 15.