You will need a disk or a bootable USB with a OS if you still want that specific OS. Otherwise, your computer might have a OS included in factory settings. To restore all your pc from factory settings, you just have to hold alt and (F10 or F11, it depend of the brand of your computer) when your pc starts (start menu), this will open a menu to clear your hard drive and go bac to factory settings. Your computer will be the same as when you started it for the first time.
What could make your computer slower,is a memory problem, it shouldn't be bad to run a memory test to see if one of your memory stick have problem or just don't work. Then you will just have to replace it.