We're having a problem at Walden School I can't yet figure out.

We have a Windows 2003 server that we've been using for many years.
Students all log on with the same ID/password to join the domain
and get access to the file server.
Today we started having problems of the server saying the
ID/password was invalid. PCs that had logged on earlier were
connected to the server without a problem, any PC trying to
connect once the problem started were not able to log on.
Happens on PCs running Windows XP or Windows 7,
connected to the network via cable or wirelessly.

After trying a couple of things, I re-set the password for that ID,
and students were able to log on again. Later in the day, the same
thing happened again, and resetting the password fixed things again.

Other logons (eg, my administrative account) work just fine.
I'm stumped for now.
Any thoughts would be appreciated.

Sigurd Andersen