Link: Your PC is NOT Old | PC Pitstop.
As I grow older (yes, I am old), I become aware that the body parts don't move like they used to. (Get your mind out of the gutter, we're talking knees and ankles and low back here). Computers, like people, get old, and they are replaced by something prettier, faster, and more efficient. I am the worst offender, in that I am always trying out a newer, faster computer. However, many of the problems with sluggish computers are caused by use and clutter, not age. A properly maintained computer can perform its functions for years. By properly maintained, I am speaking of removing file clutter, re-installing software, removing temporary files, and other techniques better described in the linked post. Before you buy that new computer, ask yourself if the old one can't be cleaned up first. As an example, She Who Must Be Obeyed asked (okay, told) me to get the school computers ready for the Fall Semester. She teaches at a small private school here in town. The computers were, on average, over 10 years old, and were running Windows 98, Windows 2000, and some weren't running at all. The solution, since new computers were out of the question, was simple. I installed Ubuntu Linux on all the machines that could handle it, and Puppy Linux on the others. Presto, the school had workable computers that, with the addition of the Edubuntu package, are designed for school children and education.