If this is builder's OEM you can reinstall using the key read from HD., if you don't have the $100+ value key backed up.
If this is factory OEM then the key read from HD will not work to reinstall - you must use the key on COA sticker, which every computer sold with Windows installed must have affixed to it.
Here are steps to get a perfect reinstall of factory OEM: Reinstalling Windows 7
You can also run factory Recovery from its partition but I would make the Recovery Disks as a backup first. Or you can run the disks if the partition won't boot to run.