Why does my wired network transfer files slowly

I have a small business network that connects 3 wired pc's and 2 wireless laptops. I am running a point of sale system on the network. I recently replaced the computer I use as a server. The work stations now run the point of sale system so slowly, they almost cannot be used. Both the wired and wireless systems are running slower. Both wired workstations are running Windows XP, both the old and new "server" are running Windows 7 64 bit. Both are Pentium dual core with 6gb of RAM. The old system has a Realtek RTL8168C network card, and the new system has an Intel 82579V network card. I replaced the computer due to other hardware issues. The cable used to connect it to the router is the same. The old machine ran the workstations MUCH faster. Does this have to do with the network card, or some other setting I am missing. I know a little about computers but am not extremely technically knowledgeable. I tried to make all the settings exactly the same so my point of sale system would still recognize the new computer.