Do i need to have the windows 7 disc if i format my harddrive?

I bought my computer my my aunt and uncle and had it for a while now and i have been wanting to format it because it is running slow and there was A LOT of junk on here before and i just want a clean brand new slate but i was wondering if i need to windows 7 dvd disc because they didnt give that to me when i bought the computer off of them.