This is one of those technology questions that a) is critical and b) has no hard-and-fast answer. In earlier times, processor speed doubled approximately every 18 months, but this is not universally true anymore. So, should you keep the old computer until you either max out the hard drive or it starts making odd noises, or should you automatically upgrade every X number of years? The answer depends on a few factors: How intensively do you use your computer? Do you have a backup computer and excellent backup system in case your current computer fails catastrophically? Do you do processor-intensive computing such as using a speech recognition program to dictate into your translation memory program? All of these factors are important in your decision on when to get a new computer.
Regardless of how often you purchase a new computer, basic digital and physical hygiene can go a long way toward prolonging your machine’s life. Especially if you have pets, a thorough cleaning of the inside of your computer’s case (make sure to unplug it from all power sources first) can be a worthwhile (if horrifying!) step, as can deleting large/old files that you no longer need. Movies and music are big culprits on most of our computers, so it’s a good idea to go through and purge the ones that you don’t want.
If you are one of those translators who likes to get the longest possible life out of your computer, make sure that you have a backup computer (i.e. a laptop or a separate desktop that is in working order) in case your computer either literally or figuratively goes up in smoke. In addition, a reliable computer backup system is critical regardless of how old your primary computer is.
When you decide that it’s time to replace your computer, it’s worth considering the option of assembling your own computer from parts. This option allows you to put your money into the features that are important to you; for example you might want a lot of processor speed but just adequate audio and video capabilities, or the other way around. For some advice on purchasing and assembling a good, affordable computer, an excellent resource is the website of computer security expert Daniel J. Bernstein. His page includes exact hardware specs so that you don’t have to figure out, for example, exactly what processor you want (plus you get to visit the domain cr.yp.to ; how cool is that?). My current primary computer is the 2004 version of Bernstein’s basic workstation and I’ve been quite happy with it.