How can I use the product key to reinstall windows 7 Home premium

I have a HP PC with the windows 7 Home Premium, I want to reinstall the windows 7 but I don't have the disc. How can I just use the product key to reinstall it?

well if you do not have the disc all most all factory built PCS have a built in recovery option to reinstall the OS to factory Default you could try that when you boot up your computer it should be like F12 or something you need to it it should be near the bottom of the screen your other option is to see if you can order a disc from microsoft sometimes if you have have a valid product key but no disc for around 30-40 bucks you can order a disc I think hope this helps.

Thanks very much! Another question, Can I download from microsoft site and install if I have the valid product key?