1) If your computer came with Windows 7 pre-installed and pre-activated, it would have a rainbow sticker at the back of the machine with a key on it. You can activate your new install by calling MS and using that key. Activate Windows 7 by Phone
The product key is stored in the registry in encrypted form, you cannot use that.
2) Go to the HP website and under "Support and Drivers
", enter your computer model no. or you can let it auto-detect. Then look for the link called "Order recovery disks". They are cheap, just $10-20.