You can use iperf for network performance tests. I suggest version 1.7.
http://dast.nlanr.net/Projects/Iperf/
E.g.
server: iperf -s
client: iperf -c
server -l 64k -t 12 -i 3 -r
This will give you a sense of the "raw" TCP/IP performance. This will factor out the hard drives, the file system, caching, and also the composite load of the drive subsystem together with the networking. It will also factor out the file transfer utility.
FTP isn't hard to set up, and comes included with XP Pro. However, XP Pro itself has some issues with native file transfer performance, and I believe this is not entirely bypassed when you use its native FTP implementation. For this, I'd suggest going to a third-party tool. I haven't really investigated these, but have gotten very good performance from FileZilla Server, at the cost of high CPU utilization at times. The FileZilla client is likely to be a decent choice (not the best perhaps, but convenient and at least passable in terms of performance). I suggest reading the docs for setup help. (To be honest, I winged it, but it wasn't intuitive, and I didn't select the best configuration; just something good enough to get by for testing.)
You also have to remember to let FileZilla/ftp go through the Windows firewall if you're using it. This brings to mind that for diagnosis of performance issues, you should try disabling the firewall and any virus scanners, ensure that you don't have any wireless perchance (which I don't think you do, but am guessing wildly based on the poor performance), etc.
The other wild guess I have is that one of the drives may be degraded to PIO mode. This would appear as high CPU utilization and poor disk performance. Tools like HD Tach can give simple and easy-to-use read performance measurements. Getting more thorough (including write performance) would be better, but starting simpler is OK as far as it goes.
So above, you have means to:
1. Isolate network performance
2. Isolate file transfer protocol performance
3. Isolate drive performance
This should give you some info.
Personally, I'm a gigabitbigot, so would jump ahead and install a PCI GbE NIC in the 10/100 computer, and add a GbE switch, but, but.. (1) There is some problem in your case that might kill that idea. (2) There are some limitations in the performance of XP Pro (which is addressed in 2003, XP-64, Vista), and so some chance of additional frustration there -- this is the problem that you could potentially avoid using FTP. There are also some settings that can help, but nothing that would bring down 100 Mb/s performance to 6 Mb/s levels as far as I know, which is why I suggest the more detailed analytical approach first.