Going from XP to SAMBA share: open a DOS command prompt and type
net use z: \\IP_of_SAMBA_Machine\SAMBAShareNAME /savecred
Type in the username and password and it shouldn't ask for it again.
Going from SAMBA to XP share: take a look in C:\Documents and Settings\ for the user you are currently "logged" in under. Chances are it is the user 'Owner.' In the Linux terminal type
mount -t cifs -o username=owner,workgroup=YourWorkGroupName //IPAddressOfXPMachine/XPShareName/ /mnt/XPShare/
...or something somewhat similar. If that doesn't quite work due to the password thing, then you can go into XPs Control Panel and assign a password to this user and try it all over again, but this time give it the password that you assigned to 'Owner.'
Hope this helps - if it doesn't, then I would suggest you look into webmin to control all of this...
Deion "Mule" Christopher