Main culprits are a virus, malware or fragmented or full hard drive.
First off download and run the latest version of CCleaner to clean up your hard drive of unnecesary clutter -
http://www.filehippo.com/download_ccleaner I'd also run the registry cleaner repeatedly until it finds no issues (i dont bother backing up the removed entries).
Check on your Anti-Virus suppliers website to see if there are any patches available that need to be manually installed then force your anti-virus to update itself with the latest definitions and run a full system scan.
Download Malwarebytes and run a full system scan -
http://www.malwarebytes.org/products/malwarebytes_free make sure you're downloading Malwarebytes and not any "free" system checking software that is linked to on the download sites.
Then defragment your drive.
If your PC is a few years old it may be worthwhile opening up the case to see if the fins on your CPU cooler are clogged with dust. The system is usually set to throttle back CPU performance if it's overheating which it will be if the cooler isn't operating properly.
If none of that works and you can identify when the PC became slow you could try a System Restore to an earlier date which might undo any changes that caused the problem.