Windows 7 is probably not installed on the drive. If HP does not provide recovery discs unless you pay an extra 10-20 dollars on the config, or online after it is bought... Recovering requires the original laptop settings, it is probably an OEM License, OEM licenses work this way:
1. An OEM copy is usually BIOS-locked to the motherboard, and will not recognize changed backbone hardware.
2. An OEM license ties it to the first computer it's installed on. A different, non-identical motherboard is treated as a different computer, and even if it weren't BIOS-locked and you could install it, you would not have a valid license and you wouldn't be able to activate it.
Basically, you will need a new license of Windows 7, You can buy one from Microsoft.com for 119 dollars if you get home premium.