For Windows: If it continues after a restart, open up Task Manager (Ctrl+Shift+Esc), do More Details, and look at CPU, Memory, Disk, Network percents. If anything is abnormally high, try sorting by that column and google that process to see if it's normal.
For more information, you can click on Performance, and Open Resource Monitor. That might help you pin down processes that are using a lot of resources.
Beyond that, you could virus scan, run something like CC Cleaner (I've heard that it can help), defrag if you have a mechanical drive, or system restore to a point before things went bad. Oh, and it wouldn't hurt to backup anything essential before taking corrective steps you haven't tried before.
For more information, you can click on Performance, and Open Resource Monitor. That might help you pin down processes that are using a lot of resources.
Beyond that, you could virus scan, run something like CC Cleaner (I've heard that it can help), defrag if you have a mechanical drive, or system restore to a point before things went bad. Oh, and it wouldn't hurt to backup anything essential before taking corrective steps you haven't tried before.
If you're desperate a factory reset should bump you back to windows 8 (if you updated from it)