If you need this only to register an installation of a software, why not generate a unique ID based on the MAC address & the current timestamp? This should make sure that for this installation instance the id is unique. I have to tell that people that know themselves in PC are able to change any two of these variables.
Combination of MAC address and timestamp gives different id when second time request for activation.
here i need to chk previously generated key and provide already given key for activation.
I know that my suggestion was not solving your problem but rather pointing out that there is well known software out there that limits itself to a certain amount of active installations. Of course I'm not aware of the specifications of your software so I'm not the one telling you what to do. How about generating a key (serial number) for a given user (who needs to register in order to obtain it) and you simply combine this serial number (encoded) with the MAC address? I suppose that's closer to what you want to do.