All you need is the license key, the actual disc it was built with is irrelevant. You can also contact the system builder and they have to supply you with a means of re-installing the machine. This can be from CD's to a recovery partition so if there isn't a partition on your machine they have to give you a CD. You can also buy what's called a media pack from Microsoft for a few bucks if all else fails. This doesn't come with any licence, its just the OS.