Reinstall windows 7 home premium

I purchase my laptop(xps15z) with windows 7 home premium 64 bit sp 1 installed in it(pre-installed).My hard disk crashed and i had to change new hard disk.Now i have to re install windows 7 home premium 64 bit sp 1 on my new hard disk.I have genuie product key but dont have installation disc.How to reinstall windows on my laptop without cd/dvd or iso file..?