How to change stored Network Printer Credentials

Try running on the problematic PC (XP), from the start/run box:
rundll32.exe keymgr.dll, KRShowKeyMgr
This should open the password management console. If your saved share is present, you should be able to delete it.
The following article outlines in detail
http://www.windowsnetworking.com/kbase/WindowsTips/WindowsXP/AdminTips/Security/RemoveSavedPasswords.html