I just pulled some hair out trying to access shared XP volumes from OS X, but eventually got it working. Here is what I learned about it, as much for my own future reference as anything:

If you use smbclient and get

NT_STATUS_LOGON_FAILED

then you have a bad username or some connection problem. However, if you get

NT_STATUS_LOGON_TYPE_NOT_GRANTED

it means that you are connecting and logging in with the right username/password, but that something on the xp side needs to be tweaked. You need to open "Local Security Settings" which is buried in the Administrative Tools control panel. The fastest way to open it is to just

Run... secpol.msc

There seemed to be two changes I had to make. First,

Security Options - Network Access: Sharing and Security for local accounts

was set to 'Guest Only - local users authenticate as guest'. It needed to be changed to 'Classic - local users authenticate as themselves.' Secondly,

User Rights Assignment - Access this computer from the network

Needs to include the user you're trying to login as. This was blank in my settings.

Also, I had some problems with the share names. Windows likes to append a '$' to the default share names, but the OS X Connect to... GUI doesn't want to deal with them. (the command line smbclient seems ok with them). I think you're best of removing all of the shares you might have, turning off sharing for each XP volume, then going back and creating new shares named without the '$'.