What happens if user will reinstall windows then .can i issue new key to user or user will re register software with exi

If the customer is using your app on the same machine, then the customer can just use the same product key to re-activate your software on that machine. The way TurboActivate works is that it locks to a machine's "fingerprint". See: http://wyday.com/limelm/features/why/#hardware-locked