This is a nice post, but I feel like a couple important pieces aren't being mentioned.

My Computer is Slow

A computer doesn't get slow from "filling up." More often than not computers that are running slowly are doing so because too much software is running at once, competing for resources. A lot of software comes with programs that run all the time. For example, your printer may come with a printer management utility that alerts you when toner is getting low, and another program that checks for updates. These programs that always run take up resources.

These days defragmenting isn't usually the answer. Instead, disable or uninstall unneeded software. Take steps to keep software you don't need from starting up. (Do you need Steam running all the time, or only when you want to play a game?) Remember that the speed loss is usually from programs competing for resources, so the other route is to get more resources. This can be done by things like adding RAM, or getting a faster drive.

My Computer is Noisy

Most computers have moving parts, and these eventually will wear out. While mechanical hard drives have their "clicky" sounds, another common type of noise is from the fans. It is a good idea to use compress air dusters every once in a while to clean out the fans. This will help their lifespan. How often to clean really depends on the environment. The big exhaust fans are often the first parts to start having trouble. When they begin making noise, it is usually a good idea to replace them.

When the fans lose their effectiveness the computer runs at a higher temperature. This can cause instability (crashes and hangs), and can cause components to burn out.