Reported Problem:
"I keep losing my network connection to the server, or to other
computers in the workgroup. Internet access is generally OK. The
problem seems to crop-up as s oon as I stop using the computer
for a few minutes, if I keep working continuously on a network document,
then everything is OK, but as soon as I stop...."
Incidence: Very Common.
Reason: Faulty powersave features on the LAN card.
Recent LAN cards (wired, and in some cases wireless) have an
option to "turn off this devce to save power." - In principle this
seems like a laudable, planet-saving idea, but in fact the tiny amount
of power saved would only concern a laptop-user on batteries - and even
then, maybe not.
Issue here is that the powersave option makes the computer disappear
from view on the LAN, and quite rightly the server assumes that the
client has packed-up and gone home for the night. Thus it closes the
file-sharing session. When the user next tries to access networked files,
there is a delay whilst the connection to the server is re-negotiated.
Some programs treat this delay as a fault, and report it so.
Resolution: Turn off the powersave option.
Go to Control Panel, and open the Networks item.
Highlight the affected connection, right-click it, and and select "Properties." You should see a pane something like this:
Press the Configure button next to the card/controller itself, near the top.

.
You'll see a multi-tabbed pane something like this. Look for a "power" or "advanced" tab, usually the rightmost.
Here the powersave option is active, so this card likely WILL give
trouble if used to access a fileserver. Untick the option.
Unless you have a pressing reason to require "Wake on LAN" then leave
the other option unticked, too. Otherwise your computer may develop a
habit of suddenly and unexpectedly springing to life.
Interestingly the dialog warns lengthily of the possible side-effects
of Wake On LAN, but completely omits to mention the problems that
powersave can cause. Which in most cases are far more of a
concern.
OK your way out, and the problem should be cured for this card. Repeat for any other network connections. (other than dialup)
There is generally no need to reboot, you should see the effect straightaway.
For an unrelated problem which sometimes gives rise comparable symptoms
- involving 'Disconnected' network shares - see this Microsoft Technet page.
In this case the fix (a registry value change) needs to be applied to
the computer hosting the share. The page mentions NT/2000 but the
advice applies to all recent Windows versions.