Anybody help?

hi,
I have three computers linked up all on XP, one of them is constantly on with lots of files on it so i can access the files from my other computers with ease. Now, the main system holding the files doesn't crash or freeze up, but sometimes the other users arnt allowed to access the file directory. I have set the permissions on the file so that aeveryone can access the files, but sometimes it still happens, is there a setting somewhere to stop it from not allowing people to get onto it?
Any help will be much appriciated.