"•Windows 7 Setup does its compliance checking before the phase of Setup where you format the disc. (Unlike with Windows Vista.) This means that you can format your existing hard drive, and blow away a previous Windows version, and not worry about activation. If it was there, Windows 7 will still activate.
A clean install is not the problem, it is whether he needs a previous OS. I can install on a blank system but I originally registered my copy as an upgrade to an old XP Home I bought back a few years ago. After that I can reinstall as much as I like becuase it is already registered.
It is still worth a try I suppose, but I did read that MS stores the original install's license.