If you haven't figured it our yet and are using Windows 10 (you didn't state your operating system). then go to the task manager and click on the User tab. It will show the CPU, memory, Hard drive and network usage. This will provide a clue to the problem.
If the CPU usage is very high in normal usage, you probably have a virus. Do an Anti virus scan which can take many hours depending on how many files your have. If you do the scan, disable any sleep mode so the computer will continue to scan overnight if needed.
Another thing to check is if there is high disk usage. Writing to a hard drive is a slow process and will greatly slow the computer if it is constantly being done. This may be a virus but also could be problems with the drive itself. If it is creating errors, there may be repeated writing or reading and you will probably get computer crashes as well. You can get better detail on the Performance tab of the task manager. You can also right click on the Windows icon (on the side of the task bar) and select Disk Management. This will show the status of all drives and partitions. They should all say Healthy.