If your disk drive failed, you would have to replace it. You would be able to use the same Windows licence for it. The worst that could happen would be having to phone Microsoft to explain that your disk drive had failed and get a new key for free.
Yep, I agree with Andy. Windows 7 comes with one license, meaning it can only be installed and used on one computer. So, as long as you only have Windows 7 on only one computer you will be okay. But, you will most likey need to activate your Windows by calling Microsoft. This tutorial will help you if you need to activate via phone: http://www.sevenforums.com/tutorials/18715-activate-windows-7-phone.html.
Microsoft Windows Outreach