Hi Arijeet Ghosh,
You can find the product key of your windows 2008 server in the following ways
a) On CD you will have the label with the product key
b) If you have downloaded from MSDN then your product key will be listed under :"download keys"
After entering the key, it would be registered under windows registry in encrypted form which is hard to read , so many vendors have come up with their own solution to break this . please find the link below.
http://pcsupport.about.com/od/productkeysactivation/tp/topkeyfinder.htm