Hard Drive Performance
Hard Disk Drives (HDD) are so often neglected. In the vast majority of PCs - essentially every PC without a Solid State Drive (SSD) - the HDD is one of the slowest components and also one of the most susceptible to break downs and issues. HDDs are meant to run for many years, which should easily cover the lifetime of a PC, but that doesn't mean you can just keep downloading movies, music, pictures, and games and expect it to be as fast and stable as ever.
The first thing I'd like to cover is read and write speeds. A HDD is a physical disk, like a small DVD that can hold significantly more data. HDDs therefore have to spin, and also have a "head" that moves across the surface to read data (ever see an old record player?). So, if a disk is spinning at a constant rate - usually 5400 or 7200 RPM - you have a situation where the larger outer portion of the disk is traveling faster than the inside. If you think of the disk as a bunch of circles, the circles will get small as you move to the middle - yet they each do a full rotation in the same amount of time. More distance traveled in the same time means it's going faster, right?
Ok, so what that means to you, the user, is that data stored on the outer portion of the drive is able to be accessed a lot faster. When you get a new PC with Windows installed, Windows itself reserves about 100mb on the outer most portion for fast access, plus all the main files are stored right next to it. As you start to add new programs they are taking up room on the outer portions, and eventually you wind up with 75% of your drive full and your newest program runs like crap because the data being accessed is taking so much longer.
The fix for this is to start using Partitions. Essentially, Windows will divide your HDD however you want and then it will consider each partition a separate drive. In this way you can store movies, images, and other things that are not dependent on speed on the slower half of your HDD, while leaving the faster half for all your programs. This is done through Administrator Tools in your Control Panel, but I suggest you do a little searching about it first to see if it's right for you.
Driver and Chipset Updates
Everything on a computer has some instructions in the background telling it how to behave. It's important to make sure these instructions are up to date, because if something comes around telling your computer to do something that it doesn't know how to do, there will be problems.
The most common driver that needs to get updated is the graphics driver. A new version is usually released every month or two, occasionally with hotfixes in between. Generally speaking it isn't vital to update every single time a new driver comes out, however, if you are planning to play a brand new game, there is usually a driver update near the release date that will improve gaming performance. For more info on driver updating, please see this blog post.
Aside from that, there are other things that occasionally need updates. If you have a new PC, one of the first things you'll want to do is update the chipset, which can increase performance and stability. The chipset deals with SATA (HDDs), USB, PCIe, LAN, etc.
Intel Downloads Page
AMD Downloads Page
Another update that can help with performance and stability is your motherboard BIOS. This is almost like a driver for your motherboard, so it handles things at the electrical level. All you need to do is go to your motherboard vendor's website and search for your model of motherboard. Usually under Support or Downloads, you can find the latest BIOS. Some vendors like Asus even have a BIOS update utility you can use that makes the process much simpler. If this is not available to you, you might want to search some "How-Tos" on the process.
There are a few programs that help find and update drivers for you. I used Driver Detective for a while. I found that it was pretty good at finding when updates were out, but it sometimes didn't actually apply the updates properly. Last time I used it, you were able to search for outdated drivers without a purchase. This can be useful, and then you can search out the driver updates yourself.
Windows also has regular updates and occasional Service Packs. You should always try to keep your PC as up to date as possible, but remember to have those restore points enabled just in case something goes wrong.
Moving My Documents, Videos, and Pictures
Related to keeping the fastest part of your HDD free of files that are not affected by performance, you might want to move where Windows defaults all your saved documents. By default, it is inside your User folder, which is on your C: drive. If you create a partition for media, or even have a separate hard drive for it, then you'll want to make Windows aware of a new default location.
Microsoft's Guide to Moving My Documents
On top of this guide, you can go to your Libraries option in Windows Explorer, right click it, and go to properties. You can then add multiple folders to each library as well as set a default.
Cleaning Up the Crap
There's often a lot of junk that gets installed on a computer, often by accident. One that is extremely easy to miss is when you install new programs, many of them ask if you want to install their "toolbar". Generally, these are toolbars for your web browser, but if you skip through the install process it's easy to hit Next without seeing that it automatically selects to install the toolbar. While they seem innocent enough, after you get a few toolbars installed it can have a mild to severe negative impact on overall system performance, depending on the PC. If you're unsure how to uninstall them, use google.
Many PCs - particularily "brand name" ones - come with a bunch of software installed. There is a specific name for this software - bloatware. In the vast majority of cases, the software is useless crap. You should open them up to see if they are at all useful, and if not, uninstall it. Sometimes they seem useful, but in fact only take over a function that your computer/Windows already has - for example, back up and restore.
Also, if you look at the icons in your System Tray - these are all the icons next to your clock in the Windows Task Bar. You'll probably find that there are a whole bunch that you have no idea what they do. Every time you turn on your PC, it has to load all these different programs that take up time and memory. Most of the time, you can select to not start the program at Windows startup. For more on this, read here.
In combination with my tips and advice on PC maintenance, you should now be able to keep your PC in a good working condition without dramatic slow downs.