computer disconnecting from AD network

One of the laptops that I support doesn't seem to be staying logged on to our
AD network. He logs on correctly while plugged in locally (the name John Doe
shows correctly when you click on the Start button), but after a time from
five minutes to an hour he seems to be logged off: the name changes to the
actual login name (johnd) and any security on network folders won't let him
access them anymore.

When he enters his login info on the window that pops up it says it is
identical to his computer login and has already tried that. When I use the
same login on another comptuer everything works properly.

This laptop was an upgrade from XP Home to XP Pro before it was joined to
the network, so I wasn't sure if that had anything to do with it.

Not sure if it's related or not, but this laptop cannot do Windows Updates
either - not even when I am using the local admin account.
.

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