client computers get logged off SBS 2003 overnight

I have installed a server running sbs 2003 standard using a domain and connected a number of EPOS terminals each running win XP Pro SP2 embedded. The software uses a mapped drive which is on the server. Each terminal is set up as individual clients with autologon enabled (each terminal has a touch screen but no proper keyboard (ie no Ctrl Alt or Del keys)
each terminal can log on and works perfectly for entire working day 0800 – 1800 approx. The problem is that the next day they have lost their connection to the server. logging off and on enables them to work once again. My question is “How do I stop them being disconnected from the server?”
The server is setup with DNS and DHCP configured