"Have reinstalled everything with all computers turned on and connected to network, with same result,ICS is working ok."
Meaning you have done a clean reinstall the OS, motherboard/NIC/sound/modem drivers?
I have a 4 computer network. 2 x XP Pro,P4-2.6 and one is P4-1.7 with XP Home and a laptop with XP Home. I have an HP OfficeJet connected to one XP Pro.
The whole system has run well for about a month, now the printer cannot be shared and the shared folders sometimes dissapear for about 1/2 hour then come back. Have reinstalled everything with all computers turned on and connected to network, with same result,ICS is working ok. Any help would be great!

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic