Spending money on a computer is not like spending money on a car. Generally you don't put a larger engine in your car, put more doors in or make it into a ute on the weekends, but that is exactly what you can do with a computer.
In computing, "If it ain't broke, don't fix it." is very, very true. When you start to spend money on one thing, pretty soon you spend money on other things and before you know it you bought a new computer.
If the computer you own is a few years old and runs Windows 98, then installing a CD burner is fine, and perhaps adding some memory or a bigger hard-drive is a solid investment, but installing Windows XP for example, is often asking for trouble.
When a computer is first bought, it comes with all manner of software appropriate to that computer. Over time more software becomes available and some of it is useful to you.
The problem is one of upgrade fever. If you always need the latest software, then you will likely need the latest hardware to match. If you bought a beefy machine when you started, you can extend the range of software that you can install, but the moment you install new applications, the more you are making your machine obsolete.
Think twice before installing the latest program.