Start off by uninstalling all the unnecessary programs on your machine. I'm pretty sure there are quite a bit. Usually when a computer freezes is because it ran out of Random Access Memory A.K.A. RAM. You might need to buy more RAM for it. Don't worry. RAM is not expensive. It also could be CPU related which also involves buying another like the RAM I mentioned above. You said you scanned for viruses so I know that isn't what is causing the slow down. Also on your taskbar on the bottom right close programs not in use by right clicking on them and selecting whatever option closes it.