it's likely you've run out of disk space. Felser has good points, knowing more about what you have will help troubleshoot. I'll assume you're running Win 10. Here's what I would do before trashing the PC.
1. Make a backup of your data. Use the default OS backup program and point it to an external HDD.
2. Reset your PC. Under Settings, Update & Security. Look for Reset this PC. Click on the getting started button. When prompted to remove personal files say yes. This will factory reset your system.
3. Relaunch windows. Test your system for performance. If it's working fine then you'll only need to bring back files from your backup. When it's time to bring back your files, do it incrementally by folder, bring back only what you need. If you save everything to My Documents you should be fine migrating all that data back. Be more careful with system files and logs - most is unnecessary junk and what might be needed is often rebuilt by the SW program. Save that backup drive, do not use it for anything until you've determined that you have all the files you want to keep.
I tell everyone who is in a market for a new PC to wait as long as possible. Spectre and Meltdown viruses are still not patched, and won't be until a new architecture is in place.
WAIT....I just saw your specs post. Windows 8??? They may have quietly re-launched the free upgrade to Windows 10. After backing up your data try to upgrade. If not, try to downgrade to Win 7. If you're able to do either select the option that resets your PC. In both cases you'll likely need to have freed up HDD space (delete your files after backup) before you upgrade. Windows 8 is full of bugs. Given you're on version 8, see if there is a windows 7 backup option on your windows 8 OS. If it's there, use that for your backup.