The first two things you need to do first are:

• Make sure you are not going to be legally liable for keeping that laptop with you even after getting let go. It’s unfair, but even if it was their fault they forgot to take it back, you might get sued.

• After making sure the first point is clear, wipe it thoroughly in case there are tracking/monitoring tools on it.

Once both are done, then do whatever you want with it. Nobody cares if you install Windows on it or dual-boot it with Linux.