On June 15th, we started getting calls from clients using Samba that had XP workstations unable to connect to shares. These were established clients running Samba 2.x. It didn't seem to affect our inhouse Samba systems, whether Samba 2 or Samba 3.
The inability to connect with the Samba shares was correlated to a Windows XP update KB2536276. This update includes changes to the SMB protocol.
The problem was tracked to Samba systems sharing with unencrypted passwords (I don't think it matters what kind of share security you use, e.g. SHARE or USER). The Samba variable "encrypt passwords" may have defaulted to "No" on older versions of Samba. In any case, the fix is to enable encrypted password shares, and making sure that the secrets.tdb file exists.
To do this, find your Samba bin folder. You should be able to run "ps -ef | grep samba" to and look at the running processes to find both your smb.conf location and the samba bin folder. Run testparm and look for the "encrypt passwords" line. If it says "No" then edit smb.conf to make "encrypt passwords = Yes" in the GENERAL section.
Newer versions of Samba should already have a secrets.tdb. If it does not exist, run "smbpasswd -a user password" to create it. Restart Samba and you should be all set.