Sometimes, there’s usually a dodgy network card that doesn’t seem to want to work too nicely in Windows XP on a domain.
The typical situation is that Group Policy fails to load when logging on or starting up. This can cause a lot of grief, and I usually find that this registry key does the trick:
Once applied, the network card may take a while (about 10 seconds) to prepare network connections. If this is the case, then you should find that the system is now working properly. Horah!