This is for a 2008 version of AutoCad LT<->
I uninstalled from my old computer and tried to reinstall to my newer machine.
This is an XP machine.
I can not get a new activation code since it tells me the serial number is not vailid.
Welcome to the community.
2008 has been retired. If your software was ever upgraded or cross-graded or ever under Subscription then that lease has expired. You probably have a newer version of LT that has been upgraded that you can still activate. See my invalid serial number link below. Otherwise, you will need to be in contact with someone at Autodesk, like philip.s, for example.
For future reference and as another option, our global License Assistance Team can also help validate a license status.