How to restore laptop back to Day 1?

My laptop used to be able to handle online games, now its **** and can barely handle 5 mins of it. The Cpu usage will skyrocket to 100 percent and that;'' be the end of it. Even task manager doesn't help. I used the disk cleaner, that didn't help much. I need to delete everything off my laptop. I hear people say you need the disk that came with the computer? >.> I wonder how i will find that. So how do i restore it back to day 1 ?