Fast user switching password problem

Not sure of the best place to ask this, but here goes...

I've got fast user switching enabled on my family machine. Periodically,
when trying to log on, XP keeps telling me I've got the wrong password
for whichever account I'm trying to log on to. I know I haven't though.
If I go to another machine and connect via RDP with the appropriate
password, I can get in. If I then kill the RDP connection, go back to
the family machine and log in again with the same credentials, it lets
me in without fail. This is not a caps problem. I haven't forgotten the
password. It just won't accept the password being entered.

I've searched google and the knowledge base and can't turn anything up
other than a few references to people with similar problems and no
solutions.

TIA,
--
David Morris
http://www.brassedoff.net/wp