Disable startup programs you don't need
Make sure there are no network shares you don't need, especially if they do not exist.
Clean out your network neighborhood.
Clean up your temporary files, web cache, etc.
Remove printers you don't need, especially network printers that no longer exist.
Choose your AV software carefully, as some of these use lots of resources
Clean your registry with CC Cleaner
Make sure no virus or malware is there with Malware Bytes
Replace old or failing hardware, as disk drives get slower before they die, and overheating motherboards or video cards are not stable.
Check for driver updates for hardware and bios updates for the PC.
Add RAM if needed.