I worked for a shady evil company, and when they illegally fired me, I never ended up returning a laptop - thinkpad running windoze.
Let’s say I wanted to put Win 8 or similar on it so I can play some video games. What would potentially be barriers to doing this? Said evil company is hopefully going out of business soon, and is a clusterfuck anyway, so they might not end up asking for it back.
The first two things you need to do 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. You also likely agreed to return all office-issued equipment when you signed your contract with the company.
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.
If OP got his last paycheck, done deal. Yes, he surely signed for it, but no laptop is worth the legal cost of a lawsuit.
SOURCE: Issued hundreds of laptops, of all varieties and prices, for multiple companies.