PC loses trust with domain overnight

We have recently rolled out XP in our environment and currently use Active Directory. It seems that shortly after implementing Active Directory, we occasionally got a PC that lost it’s trust with the domain (single domain environment). The person would use the machine all day and shut it down, then come in the next day and couldn’t log in because the machine lost it’s trust. This happened to one person two days in a row. We don’t see it too often, but when it happens, it is with a machine that had been on the domain within the last 24 hours. Any ideas?