Along with page about keeping your computer secure that I usually leave with people that I've fixed up their computer, there's this page which discusses other things related to keeping your computer running smoothly, that aren't really to do with keeping it “secure.”
Configure your system to work sensibly. You probably do not need your word processor, music player, nor a pile of other programs, to start up every time you use your computer. As you add more software, on Windows computers (at least), you tend to get a collection of things starting up unneccessarily. Turn off options that start them automatically every time you log on.
Remove programs that you never use. If you don't use it, you don't need it. They're wasting space, and will be one less thing that might be the cause of any problems.
Regularly make back-ups of important data. Computers are not infallible, nor are their users, files do get lost. Make back-ups of anything that you need to keep, make more than one back-up, and store them separately. You should, also, check that the backed-up files can be read back.
Keep your files organised. When saving data, do it in a way that makes it easy for you to find them, later. And do it in away that you can still manage when you've got a large collection of files. e.g. Name files and directories sensibly, and use different directories to group files that go together. Also delete things that you don't need anymore.
Most personal computers need to have their drives defragmented regularly. This may be an automatic feature, or might need manual intervention. How frequently you need to do this is variable—it'll depend on how much spare drive space you have, the filing system that it uses, and what sort of things you do with your computer.
When computers write to their disc drives, they write data in whatever spare space is available on the drive. This space isn't always in one continuous lump, but is often a collection of individual small spaces scattered across the drive—the space taken from where other files used to exist. This is called fragmentation.
There's several reasons that fragmentation happens, here's just a few examples: If you delete one of your own files, that leaves a space behind, in the middle of everything else, and that space will be used to save something else, later on. Or if you re-open an old file, edit it, and save the changes, the file has become bigger, and the extra parts will be saved wherever there's spare space. And various computer programs save and delete temporary files as they work, even if you're not aware of it.
Fragmented data takes more time to read than unfragmented data, the computer has to collect all the parts together that are spread over the drive, this makes the computer seem slower. De-fragmenting (aka “defragging”) re-organises the contents of the drive, putting all the pieces back together, and speeds up use of the disc drive, which often speeds up the whole computer.
Batteries in laptops and uninterruptible power supplies should actually be used. Rechargeable batteries work better, and last longer, when they're used to power the computer periodically, then recharged. Batteries that are left flat, or continously charged without being used, tend to get wrecked, sometimes quite quickly. For a laptop, you should occasionally run the computer solely off the battery, and it's better if you do so until the computer warns you that battery is almost empty. Maintaining UPS batteries may require a more complicated regimen, especially with computer systems that must run continously, though a good UPS may manage that all by itself.