Thanks for your help. Here is a copy of the event log that had an error at about the same time as the problems occurred.
Event Type: Error
Event Source: NETLOGON
Event Category: None
Event ID: 5719
Time: 4:12:05 PM
No Domain Controller is available for domain THORPE due to the following:
There are currently no logon servers available to service the logon request. .
Make sure that the computer is connected to the network and try again. If the problem persists, please contact your domain administrator.
For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp
0000: 5e 00 00 c0 ^..À
I looked up the Help and Support center link and it suggests changing the registry and making sure the drivers are up to date. Although I'm not excited by the idea of changing the registry, I'll try it and see.