Thanks for your answer Jim

What does it do (or not) when it "hangs"?: The system stop responding, ever the mouse pointer is stopped

Are the 3 PC connected to the network?. Yes, but without the IPV6 stack, source of problems.

Are they the only ones with XP Pro, or are there others?. all three have XP pro.

Is there virus software active? (workstations and/or file server). Yes, but on the reinstalled unit it is not installed.

Are there any other PCs with the same integrated video that are working OK?. No, all three have the problem.

Answers to these may help with ideas.

Thanks a lot Jim.