Erasing data files, changing system passwords, or worse, installing a computer virus into the system, are all problems I've handled in the past. I've even heard of an employee stealing a firm's file server. Two key points are prevention and backups. If data files are erased, there are several methods of recovery. The easiest and cheapest solutions involve having a backup system in place.
Passwords can also be a problem.
Usually there are two or three levels of password protection in accounting systems. WordPerfect documents can also be locked with passwords. Computer viruses are programs that reside in the computer, usually in the RAM memory. There are several excellent computer anti-virus software programs on the market that will take care of the virus situation, both detection and removal. There are varying levels of DR planning.