Network Major Malfunction!

I am not sure if this is a hardware or software problem, at this point. I have a program that runs in a server/client setup. I have computer A as the server, and computer B as the client. Software is installed correctly by the book. Drive C on puter A is mapped to puter B. All files are accessible on puter B. I can open program on A with no problem. If I attempt to open the program on puter B, I lose the network connection between A and B. I can still access all other machines on the network from A, and B. I just can't connect A to B unless I reboot A. I installed a new NIC in puter A. It made no difference. I loaded another machine, puter C, with the program, as a test bed, to replace puter A (server). Same network, same cable, same identical config, including puter network ID, etc, on puter C. Puter A was removed from the network, as well. Program opens on puter B, and functions as designed, with puter C as the server.
Puter A: NEW machine. AMD 64 3000. ASUS m/b with lan, 1 gig PC3200, 160 gig SATA drive. XP Pro (not 64 bit version)
Puter B: NEW machine. AMD 3000 Sempron. 1 gig PC3200, 160 gig SATA drive. XP Pro.
Puter C, test bed: old machine, AMD 2000+, 512 DDR?, 40 gig IDE drive. Win 2K.
Puter A has been re-formatted and only minimum software installed as required to operate.
Just a question as a sidenote- is XP 64 bit version OS required to use AMD 64 processor?
But all in all, I need to get this issue resolved ASAP. Ideas are appreciated.

 

OK it appears I have been able to resolve this issue. I d/l the NIC driver from ASUS website. I updated the driver in control panel. It said the same driver details as the one I replaced, but now it works. Go figure.