It's pretty clear why viruses could slow down your computer - they can hijack anything, from programs to files to your browser and show ads, report private data, or delete your personal information. Viruses usually take up a lot of PC processing power and will make your computer more and more unreliable over time. To remove these viruses, download and install an antivirus/antimalware solution and run a scan that quarantines threats.

Many programs create a startup entry for themselves once you install them. As these startup entries pile up, the computer has to dedicate more and more processing power to these startup demands, which means less power is left to load the desktop and open applications, especially during the first several minutes when you've turned your computer on. By disabling startup entries in Task Manager and the System Configuration utility, you can drastically improve startup time for your computer.

Windows operating systems generally start to slow down when your hard drive is over 90-95% full. With this in mind, it's a good idea to purchase an external hard drive or thumb drive and move your biggest files - usually contained in picture and video folders - over to that external drive so your hard drive has some free space. Deleting programs you don't use is another great way to clear out some hard drive space.

As files and folders get written, edited, and deleted over time, parts of some files get spread out over different parts of the hard disk. For example, if you install a new program after deleting a few files, the new program will take up data in the old data blocks that the files were in, plus any additional blocks on other parts of the hard drive. When programs like this are run, Windows has to constantly jump back and forth between different parts of the hard disk, slowing the process down entirely. By defragmenting your hard drive, Windows 

Various temporary files, such as cached internet webpages, the Recycle Bin, log files, and thumbnails are stored by Windows even though those files aren't necessarily important. Clearing these files out using the Disk Cleanup utility can moderately speed up your computer and often save you a good amount of space on your hard drive as well.

When you search the start menu, Windows searches results from an indexed database to increase the speed at which you receive results. However, this can be counterproductive if Windows is set to index the entire C:\ drive or various folders (such as AppData) that usually don't get searched yet take a long time to index. By opening the Indexing Options dialog, you can allow for indexing on your Users folder without searching the rest of your computer - maximizing the speed of both your start menu searches and the rest of your PC's functionality.

Windows has always been an operating system known for its unique UI customization features. Especially from Windows Vista on through 7, 8, 8.1, and 10, appearance features like animations, fades, transparency in Windows Aero, and smoothed fonts can cause your PC to run a decent amount slower. Disabling these effects under Performance Options can provide a much needed boost in your PC's speed without requiring you to upgrade your hardware.

Google Chrome isn't the lightest program when it comes to your computer's resources, specifically RAM usage. Each tab and extension requires more hardware usage than most of its competing browsers. This doesn't mean you have to ditch Google Chrome, however - by disable extensions you don't use - especially search engine extensions that are usually adware anyway - along with disabling push notifications, and disabling the background Chrome service altogether, you can tame Chrome to become a much more manageable beast on your resources.

Some background services, such as Plug and Play or the DNS client are very important and critical for Windows to run stably. Others, such as ones added by programs that you've installed, might not be necessary. Such services can be disabled in the System Configuration utility. It might even be a good idea to disable all non-Microsoft services for a clean boot - probably the easiest way to boost your speed in just a few clicks.

So you've removed startup entries, got rid of unnecessary programs, cleaned up 200 infections in Malwarebytes, and tweaked some other settings, but your computer is still crawling. Now what? The honest truth is, no matter how well you clean your computer, old hardware is the ultimate limiting factor when it comes to speed. If you're trying to run Windows 7 or 10 with 1GB or 2GB of RAM, it's definitely time for an upgrade. I recommend an absolute minimum of 4GB RAM to get by in 2020, and would highly recommend purchasing 8GB for the extra breathing room, even 16GB for the small extra price if you can afford it.