Removing credentials/permissions from a drive mapping

I have a user who maps a drive into a Linux server from a WinXP machine. Recently he’s had issues being able to make modifications to files in this share. He has disconnected the share, rebooted and browsed to the same location (his PC does not re-prompt him for a username and password) to no avail. However, he’s able to go to another XP machine, browse to the drive and work with no issues.

It seems that his PC has stored or cached his credentials for this server somewhere, but I can’t recall where they would be kept. Can anyone help?