Why does my computer keep lagging?

Iv'e had a W8 Lenovo laptop for a couple of months now and it has always been a little laggy, but recently it has started to go totally nuts; programs crash all the time and freezes. I really don't have high standards but even I can't take it when moving the cursor causes lag. Iv'e done multiple searches with several anti-virus softwares and cleaned my hard drive every day. Iv'e even tried to delete everything and start over. Please Help ME! I can't afford a new computer or reinstall windows. Is there anything that can fix this?