Windows XP Pro file sharing problem

I have a work station running Windows XP Pro SP2 in a domain network. I am or rather was using this work station as an extra file-server.
This has been working fine for 6 months until today. The folders are still shared and can be seen on the network, but no one including administrator can access any of the files or the network. They can’t even be accessed from the domain server. I tried re sharing the folders and even rejoining the PC to the domain, but nothing seems to work. It keeps saying this folder is not accessible and you may not have permission to use this network resource.
Short of reinstalling Windows I am out of ideas. Has anyone seen this problem before? All ideas or solutions are welcome.
Thanks in advance