AutoCAD 2000i Authorisation Code is not

One of my AutoCAD 2000i user was using an evaluation version on his PC and by the time I recieved the Aythorisation code it was more than one month and the user tryed to change the system date and use the software with a back date, but the system rejected the attempt and stoped the software, the problem is when I recieved the code system is not accepting it and giving a Registration Error, I tried the same Authorisatin Code on a different system and it is working fine, we have 50 user lisence for the same code, I know that there will be some code on the PC which activated when the user tried to change to an old date, any way to find it out and clear it.

Thanks in Advance