One Server - File - Two Users Editing It, Third Errors Out

There is 1000MBPS network set up between three computers and server.

Server has files which program saves.

Program is being run in three different computers.

Each computer tries to get into same file (eg last bill) that was created before. First computer goes in within seconds. second minutes and third just crashes. No matter which computer runs it first, last one crashes.

So called "server" is running on Windows XP pro since it just needs to keep one program files. However I cant find out whats causing file sharing over network bug that crashes access to same file if connection from many computers.