Stop Mapped Drives Disconneting

Run regedit from the machine that contains the drive
Go to...
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters
Here, you will find the autodisconnect "idle time" value of 15 minutes.
You can double click on the autodisconnect entry, select decimal, and enter the desired number of minutes (up to 65535 minutes on server 2k).

It is possible that this is not the issue. If this doesn't help, please respond.

It could be your clients...

Go to your Network Connections in the control panel.
Right click the LAN that is used - select properties.
Click the configure button beside the NIC on the top right.
Select the power management tab.
Remove the check mark that says "Allow the computer to turn off this device to save power".

the way we fixed this is to got to Local Security Policy under Administrative tools and set the following

Open Local Policies, then open Security Options

Look for a line in the right hand pane called 'Amount of idle time required before disconnecting session' double click on it and change the time to 0 (Zero) it should then show 'do not disconnect clients' above to time box

for some reason you may have to go through this process a couple of times before the change is accepted. we have done this on all our win2k servers and workstations and the problem disappeared

hope this helps

Run regedit
HKLM\system\CurrentControlSet\Services\lanmanserver\parameters
edit autodisconnect change value from F (15min) to FFFF (65,535 min) for XP FFFFFFFF.