If you have your original disks and/or keys, you can likely re-install the software necessary to get the computers back up and running. I would question whether it would be worth buying new licenses for the computers, as the value of a new Windows license likely exceeds the value of the computers, system software installed, on the used computer market.
I have no way of commenting on what value, if any, existed in your personal files. It would seem odd to let a co-worker use a computer for a year if it had valuable personal files on its hard drive, let alone without backing up those files. For the computer you used, if the personal files had not been touched for three or more years, it would not seem that they had much value. If the computer was being used for personal purposes once put into use for the benefit of your employer, the question is raised of why (particularly for a manger of operations) that should be treated any differently than personal files saved on any other work computer.
Before you can decide if it's worth trying to pursue this, you need to figure out a dollar figure for damages that you might be reasonably able to prove in court. Based on that figure, you would have to evaluate whether it's worth the trouble of trying to convince a court that your ex-employer acted improperly.

