Fog,
It's not a bad idea to replace your onboard NIC with a PCI card. May as well get a 10/100/1000 while you are at it. I always prefer cards vs onboard devices unless I don't care much about sound quality.
The only thing which argues against it would be that a reboot fixes it. Which makes one think of a RAM issue. Have you checked in Task Manager to see what is using what? I don't think it is the computer unless you are having an issue with RAM. Again, what is your configuration again? You simply have it running on the main computer shared out and all the other computers are running XP Pro?
If none of these things help, message me; and I may have a suggestion.
