To obey the law you would buy a valid license key to install Windows 7 to a new system plus, obviously, the install media (DVDs).
If I recall the Microsoft EULA correctly, you need a "full" version of Windows 7. That is, a version which can be installed to a system with no (Windows) operating system installed. You are not allowed to use a (potentially lower cost) "update" version of Windows 7 in a Virtual Machine.
If what you are thinking is to somehow track down the files on your HP laptop to recover/reinstall Windows and use that as the install media for a VM, that is even less likely to work. The recovery tools on your laptop almost certainly will check for the specific hardware context of your laptop. Your virtual hardware will look nothing like that and the tools will refuse to install Windows.