Copy of Files Across Network Repeatedly Fails

I currently have two computers on my LAN: 1) a laptop running XP Pro,
w/ integrated Broadcom wireless (g) and 2) a desktop running 2000
Server, w/ a wired NIC card. They both connect through a D-Link
router.

From the laptop, I connect to the desktop through a NET USE command.
When I try to copy files from the laptop to a mapped desktop drive
(either by dragging and dropping or through a command-line job), the
transfer fails after a few minutes of copying files. I get a message
to the effect of "Network name no longer available," although the
mapped drive still shows up under My Computer and continues to be
accessible if I double-click on it.

My backup jobs are meant to transfer about 5-10 GB across the LAN, but
the overall size doesn't seem to be an issue, as the transfer fails
after only 50 MB or so has been copied. I am running Zone Alarm for a
Firewall on the laptop, but I have configured it so that it doesn't
block traffic back and forth.

Any ideas as to what might be causing this problem? Any help would be
appreciated.