Cant access as400 SMB shares on Windows XP

We have Got some users who can access as400 SMB shares when using Windows 7 fine when using the UNC path eg \\as400 but we have got users who use Windows XP and cant it constantly prompts for credentials and even after entering the correct creds it locks the account on the as400 I have been into the local security policy on the XP machines and had a look at the LAN manager authentication...