Can some one help me with my gateway laptop?

I just bought this gateway laptop from an old friend about a year ago at the time he had boughten it three months before he sold it to me I know this because was there so i now it was legit but i have had for about 8 months or so now but now every time i turn it on it says some Copyright information then under all of that it says no bootable device--insert boot disk and press any key, but when we bought it it didn't come with one we bought it new from best buy so i'm know it was legit so what should i do can a buy a boot disk or do i need to call gateway i would but every thing is regesterd in his name and i don't want to call him and ask for the information and i don't want to mess up and them register it as stolen so what should i do? any helpful information is greatly appreciated.