Slow Workstation Performance – Need some suggestions

Hello – I currently have a relatively new network that was installed onto old cat5 cables that I am confident were not installed with interference protection. I have 3 users out of 40 who suffer extreme network slowness. I have reimaged all three computers, verified settings are accurate, revised drivers and installed two new network switches. I have installed a small switch on one machine and it seems to have helped the user some. It seems however, this issue is still not corrected overall. I am at a loss for idea’s, anyone have any other than looking at rewiring as an option?

Network Note: 1 SBS 2003 Server R2, 2 HP Unmanaged switches, workstations are Windows XP Pro – Svc 3.