Computers are expensive, so it makes sense to take good care of yours. Ideally, you'd take great care of your PC from the beginning -- cleaning it regularly with canned air, being careful with it, and protecting it from power surges, heat, and drops or falls.
But maybe your PC is already old -- so the "take good care of it from the beginning" ship has already sailed. No worries -- you can still get some more life out of your PC if you follow these steps.
Clean It Out
Dust, pet hair, and other debris can easily accumulate inside your desktop PC or laptop. If you have a desktop, keeping your tower on or near the floor will cause it to accumulate more dust, but even if your computer is never anywhere near the floor, dust and debris can build up inside over the years.
You should open up your desktop PC or laptop case and clean the dust out, but be careful. Use a small paintbrush with natural bristles, as natural materials don't build up static electricity -- and a jolt of static electricity can fry your motherboard. You'll also need a vacuum cleaner, but take care never to touch anything inside your PC with the vacuum wand or hose. Instead, hold the wand or hose just outside the open PC case and use the brush to carefully brush dust and debris towards the wand. You may not be able to get the inside of your PC completely dust-free, but focus on removing dust balls from around the fans that cool the CPU. If you have to touch any part of the inside of the PC with your hands, set down the brush and vacuum cleaner wand and tap your fingers against the PC frame to release any static charge you have built up first.
Protect It from Heat
The older your computer, the more likely it is to overheat -- and if it gets too hot, there goes your graphics card and CPU. Make sure the fans have proper airflow and keep your computer in a clean room. Keep it out of the sun, and keep the fans clean so they can cool the CPU properly. You can also add extra fans if you need them.
The biggest reason why older PCs slow down is because their random access memory (RAM) and hard drive space is no longer adequate to the user's needs. It's pretty easy to upgrade your computer's RAM, which can make it run faster and last longer. You may also want to consider adding a solid state drive (SSD) as a primary boot drive, or as a replacement for your aging hard drive -- you'll find that your computer boots up much faster from an SSD. SSDs are getting very affordable -- you can pick up a new 250 GB SSD for less than $100 -- and most PCs have slots to accept both a traditional hard drive and an SSD.
Don't want to start replacing or adding parts? That's understandable. You can and should still use Disk Cleanup to defrag and remove unnecessary files once a month or so. For a more thorough clean, use an app like Cleaner One Pro for Windows to get an overview of which files and apps you no longer use or need.
Don't Ignore Your Updates
Software updates contain patches for known security flaws that developers have discovered in the OS, but they also contain fixes for performance issues that have cropped up with the OS. You can set updates to run during the night, so they don't disturb your work. Make sure you keep your antivirus software up to date, too.
Be Careful with It
Try not to drop or jostle your PC, especially while it's running. Handle your laptop by the base, not the screen, and try not to move it more than necessary. Use both hands to hold your laptop, and when you're traveling with it, carry it in a padded laptop case. Use a surge protector to protect your PC from power surges, and for extra protection, unplug it when it's storming outside.
When you take good care of it, your PC can last a long time -- and the longer it lasts, the longer you can put off buying a new one. Even if your PC is already old, you might be surprised at how much life you can squeeze out of it.