Just thinking out loud and I'm no network expert. I have worked as a
computer tech for periods in my life, however.
The first thing is to isolate the problem as to whether it's a
physical connection problem or whether it's a data problem. The
obvious is to check the LED's at the NIC, assuming the NIC has them.
On my system, there are two lights where the connecting cable plugs
in. One indicates a correct physical connection; the other indicates
network traffic. Check the cable itself.
Data transmission between machines is also highly dependent on good
grounding. I had an actual case once where I was working on a problem
over a telephone line between cities 500 miles apart. It turned out to
be a badly grounded electrical outlet in the remote city. The third
connection in an electrical circuit is a bonding ground and normally
connects to metal parts like a chassis. If the ground is not stable,
it can affect data signal levels. I mention this only because the lad
upstairs is likely to be plugged into another outlet than the other
computers, and grounding might be an issue..
If it's a data transmission problem, it could be related to one of
those mysterious computer lock ups. Electronic logic involves a chain
of dependencies. If part of that chain locks up, for whatever reason,
you can't always reset it by powering down and back up again.
Sometimes you have to actually unplug every power connection
physically to reset it. Logic devices actually jam up at times. There
are still voltages available to them and they wont reset. Unplugging
the power cord removes all voltage and they will reset.
I was working with a guy once installing a computerized phone system.
I left him to install the main switch, and every time I passed him he
was swearing at it because it wouldn't work properly. Every time I
passed, I told him to unplug the power and plug it in again. The more
I did this the more exasperated he got. Finally, he swore at me and
asked what bleeping good my suggestion would do. I just grinned and
said, "humour me". He did, and voila...up came the system. I've seen
this happen countless times and there's no really good explanation for
it.
It may help if you remove the NIC drivers on the buggy machine, remove
the NIC and reboot. Then reinstall the NIC and it's drivers.
Apparently this action with XP will reset the entire network. I've
seen it work too with 98SE networked to an XP machine.
Is there a possibility of an IRQ conflict between the printer and the
NIC? Normally, the printer port is LPT1 on IRQ7. There are normally
two port addresses associated with that and I can't remember them. But
you could look that up under System/Device Manager. Hardware conflicts
will show up in there depending on the view used.
Finally, have you checked all your IP settings, assuming you're using
TCP/IP? Sometimes they change for unknown reasons. I know there's a
default IP XP sets if it can't find the DHCP IP. If using Xp, check
the Event Log in Administrative Tools.
