download windows 7 home basic

hi i have a lenovo g560 laptop
core i3 processor
500 gb hdd
2 gb ram

i have a serial no. for windows 7 home basic but didnt got any cd for reinstalling if i want to format my laptop

please tell me how to download win7 so that i can format my laptop and reinstall my win7 home basic......

waiting for a rply
thanking you