I'd use ccleaner to clean your hard drive, make sure your anti-virus is up to date and run a full scan, run malwarebytes full scan. Then run Error Checking by opening explorer, right click the c: drive, choose properties, on Tools run Error checking. It may require a re-boot. If that turns up any problems I'd download the hard drive's manufacturers diagnostic and run that. If the disk shows no trouble I'd defrag it anyway. Good luck.
thanks ill try some of it
grab an ssd and reinstall windows if you don't want to go for an ssd than just reinstall windows, your running on some old hardware as well so it's to be expected maybe it's time for an upgrade
if it only lagged in diablo 3 then yes its hardware, but its not time for a hardware upgrade when windows itself is lagging...