I would get a copy of Windows7 somehow, its just that much better.

Else I would run perfmon and look and see what your processor utilization is and possibly paging. My guess is that you have some affinity issues.

I would also make sure your sound settings are disabled (versus just muted) for the slave machines and obviously any voip as well, since they have a thread of their own. If you have a separate audio card, I would make sure your using it as well to off load that.

My guess is that perfmon is going to expose a race for two processors, and the other two process are going to be substantially less used.

My hope is that your ATI setup is up to date, a lot of updates have come down that are quite huge for speed improvments.