I've been in IT for about 15 years, helping individuals and handling small business networks.
I've worked with Quickbooks on-and-off over the years, usually setting up the multi-user end of it, but I'm a jack of all trades.
According to Quickbooks docs, there's a tool and a few things to try to ensure the company file doesn't have corruption, but if these things fail the file will need to be submitted to them for repair(for a fee, of course).
As far as protecting the laptop without a Windows password, most laptops will allow you to set up a BIOS password.