pspinku,
ID File generation is well described in the online help in Domino Administrator.
The short version is that the ID file is created when you create a user in Domino Administrator.
A good rule of thumb with Lotus Notes ID's is to also store the original user ID File in a safe network location (back etc). This way you will always have a copy of the orginal user ID file with a known password. Use a copy of the original for the new user.