There are a few basic things to try.
Have a look in task manager to see if any specific processes are hogging the system.
Can you remember the pc running well before any specific programmes were installed? If so you could try a system restore until before that date. If you do this anything installed after that date will need to be reinstalled.
Uninstall any programmes that are not needed.
How many anti-vurus programmes do you have running - they may be conflicting.
Are all of the updates up to date?
All all drivers up to date?
Have a look to see if you have any add-ons in your browser which you don't need any more - conflict with these often can cause slow running. Often the only way to find out is to disable them one by one until you find the possible culprit.
It could be that there is some malware - try downloading Malwarebytes (the free copy) and see what happens.