The first thing I'd do is open POS Configurator, go to the Employees tab, and then the Employee Status tab. Put in a date in the "terminated" area to tell the system the employees are no longer active.
I don't understand what you mean by 12000 hours but by making them inactive, that should solve your problem.
Updating the service pack never hurts but if it's a configuration problem, no update will solve your problem. Concerning your computer freezing, often customers don't delete old reports and files. Delete what you no longer need, run a de-fragmentation of your hard drive a few times.
Hope that helps!