It's an amalgamation of you, Microsoft, your original computer manufacturer and needed software.

You or family members download app's (programs) and don't check to see what comes with the program or see if it's going to run at startup.

Microsoft keeps updating your original program with "fixes" that make it safer and take more memory.

The original manufacturer knew Windows and your computer would run with X amount of memory so that's what they gave you, now you may need twice as much memory and your video was basic while today you're watching a lot of video that requires more memory.

Lastly like most people have Adobe products that load on startup and "give" you extra programs, the same goes for JAVA and no doubt some others.

The cure?
Go to msconfig and uncheck everything that isn't antivirus.  Go to Crucial.com and get as much memory as you can put in your computer.  Run the system tools on your hard drive that check and fix the drive.  And if you deleted programs rather than uninstalling them go get Iobit uninstaller, reinstall the program and then use Iobit to completely remove the program.  Lastly if your hard drive is actually more than 70% full buy a new drive that's bigger and put that drive in or put in a SSD drive if your current drive is only 30 ro 40% full but slow.

I suppose it goes without saying that you've already run Malwarebytes and a couple (2) good antivirus programs to make certain your computer is clean.