Try shutting it down for 15-20 minutes to let it cool down and clear memory. Then reboot and see if there is an improvement. If there is, then its likely a heat related issue. In which case, get rid of the dust that clogs the fans. Do that anyway.
Then, do a defrag, clear out unneeded files before defragging using the tool supplied by windoz. Reboot.
Try a free online virus scanner. Don’t use the one in your machine unless you have that scanners boot disk with the scanner able to scan in dos mode. Make sure your machine is set to boot from disk.
Those are the basics.
Thanks. Win8 (v.3 with updates) doesn’t let me defrag. It tells me it will notify me when it is needed. Same with chkdsk.