Check for viruses and spyware
If your PC is running slowly, it's possible that it's infected with a virus or spyware. This is not as common as the other problems, but it's something to consider. Before you worry too much, check your PC using antispyware and antivirus programs.
A common symptom of a virus is a much slower-than-normal computer performance. Other signs include unexpected messages that pop up on your PC, programs that start automatically, or the sound of your hard disk constantly working.
Spyware is a type of program that's installed, usually without your knowledge, to watch your activity on the Internet. You can check for spyware with Windows Defender or other antispyware programs.
The best way to deal with viruses is to prevent them in the first place. Always run antivirus software and keep it up to date. Even if you take such precautions, however, it's possible for your PC to become infected.