Reposting with updated information:

I have a Dell machine with Windows 2000 SP4 (all updates) and the latest
NIC driver (from DLink's Web site) has a problem with the NIC connection
everytime the computer is powered on (not power management) and Windows
2000 is loaded. The NIC is not found. We tried moving the NIC to other
USB ports (front and back), but we can't seem to configure the network
connection and the driver. NIC seems to like the same port to work.
Rebooting the computer mostly works (75% chance, it works).

In order to fix the missing NIC problem, he has to unplug the USB NIC
and replugs it in. Then, Windows will find it and network connection is
established without any incidents. This is very annoying. When a reboot
occur and network is not established. Windows' Device Manager show a
yellow "!" for this NIC when it is not working. Disabling it and
re-enabling it. Note the two different symptoms: Power off and power on:
NIC is not found. Reboot (75% success without problems): NIC is not
working and Windows' device manager shows "!" (workarounds: reboot
again, unplug and replug, OR disable and re-enable NIC in device manager).

The Dell machine is Optiplex GX 260 (P4 3 Ghz). The OS is not OEM. I
tried removing the drivers, upgrading Intel 82801 USB Controller for
Windows 2000 SP4 as noted on
http://support.dlink.com/faq/view.as...stion=DWL-120+
...., etc. No luck.

The weird part is that there were no problems in the beginning and then
all the sudden one day, this started happening ever since (three weeks
already).

Any ideas why this is happening and how to fix it? Thank you in advance.
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The symptoms you are describing are erradic enough to sound like a hardware
problem to me. If the issue just started occuring with no hardware or
software changes I would definantly begin to suspect hardware, particularly
due to the erratic nature of rebooting and removing and adding the device
to get it to work. My bet is eventually the hardware component thats
failing will stop functioning all together. Next time it has an error and
you reboot and it works check out the setupapi.log and scroll down to the
very bottom. Setupapi.log is where Windows would record the process of
adding this device and once in there look for error codes and such.