I've disconnected the router and internet connection, etc. to keep it simple.
There is one 10/100/1000Base-T Gigabit switch. D-Link 1008-D. All cables are normal (not crossover) CAT-6 and they are all less than 20ft. long. There are three computers connected to it. Two Macs and one PC:
2 Macs - Both are G5 dual processor 2.5ghz with built in gigabit ethernet. Both running OSX Panther 10.3.5.
1 PC - P4 2.6ghz MSI PT880 motherboard with a built in Via Gigabit ethernet adapter. Windows XP Professional SP-1. Clean installation was just made with BIOS and drivers all updated.
The computers all see each other just fine. This was never an issue. Furthermore, transferring between the two Macs is fine- I get gigabit speed. The problem is that transfering between the PC and either of the Macs, I get very, very, very slow transfer speed. About 10Mbps or 1 MB per second.
This has been the case whether or not the Macs have Appletalk active or not, and whether or not Windows sharing is active or not (with Windows sharing inactive, the Macs can still see the PC).
The PC's motherboard came with a utility called "Via Gigacheck" which seems very complicated and advanced. Runing the diagnostics, it says everything is A-O-K and that there is a good quality Gigiabit connection between the PC and the network switch.
The switch shows green lights for 1000 connection, and amber lights for 100 connection. All the lights for all three ports are green. So there is a gigabit link connecting them all.
Please do not respond with nonsense about the hard disks or the PCI bus on the PC creating a bottleneck. I've been testing RAID0 to RAID0, but even the slowest hard drive on the planet at this point is still faster than 10Mbps. Furthermore, the PC has the Gigabit adapter built into the South Bridge, which means that it does not go through the PCI bus.
I have wasted an entire work day trying to figure this out. I am desperate. Thank you. I've been saving these points for something very important and this is it.