Have you tried a Virtual Machine
If the problem was software only then a virtual machine running on Linux is the obvious answer, because NETBEUI is involved you will have to try it to see if the performance is good enough (it probably will be). Because this is a business the choice of Linux distros is simple, You should use a Redhat Enterprise Linux clone like CentOS or Scientific Linux. RHEL is supported for a very long time, 11 years, and it's incredibly stable, it just doesn't break. If you want to clone the existing system you follow the same procedure that you would to create recovery files in case of a disk failure. Using something like Acronis, or any other XP backup tool, to create the recovery files. Then you need to create a fresh KVM virtual machine on your Linux box, there is a very simple GUI to do this with. You should set the MAC IDs on the virtual NICs to be the same as the MAC IDs on the system that you are cloning. After you created the VM you should attach the ISOs of your recovery disks as virtual CDROMs. Then boot the VM and follow exactly the same procedure that you would if you were restoring to a fresh hard drive in a real PC. Once you are done you can boot the VM and you will have a clone of the original machine. If you have an install CD for XP you could also just create a fresh XP virtual machine.
Once you have the VM you won't be tied to any particular piece of hardware, you can move a VM anywhere, it's just a file. As long as you make sure that the VM on the new machine is a clone of the original (same MAC IDs, same virtual graphics card), XP won't know that it's been moved. This will allow you to use the system forever even if the PC dies, you can always move it to a new PC. You also won't have to worry about the system becoming corrupted, as long as you keep backups of the VM file you can restore the system. If you keep a copy of the VM on the same PC then you can restore the system in about a minute, all you have to do is rename the backup copy to the primary VM's name and reboot the VM.
The underlying Linux box won;t be subject to viruses, it will be much more stable than any Windows system. Linux includes SAMBA so it can share directories with the Windows systems on the network, including the XP VM. If someone needs to access the Internet from that box they can do it from Linux so it won't be subject to any malware. I've been using WIndows VMs on Linux since 1999. I've never had a virus on my WIndows VMs, not even when I had a Win98 VM, because the only Internet access I do on the VMs is to a couple of trusted sites like Microsoft and Intuit. I use Linux to access the Internet and it's immune to the malware that's out there including the stuff on sites of a sleazy nature.