Can't Access Shared Files On Server From Network Computers

Have a server linked to four computers and a laptop for a business and can't seem to access shared files on server from other computers as it keeps asking for username and password to access it. Have tried to change from workgroup to domain that the server is on so to create username and password but error message keeps coming up 'a domain controller for the domain ata.local could not be contacted or if just use ata as domain it does not accept any username or password. Also have tried suggestions in other forums such as simple sharing and creating an account on other pc's with same username and password as server. All of the systems are windows xp and the server is windows standard.