I have several PC's, hooked up to a 100 MBits standard router/internet
gateway by means of RJ-45 cables. (IP adresses in the range
192.168.0.xxx).

On the other hand, I wanted to transfer large files between two PC at
higher speed than what 100 MBits Ethernet would allow. I tried to
connect two PC's with a firewire cable. It worked really good. Windows
XP took care of the settings (I think he assigned IP adresses in the
range 10.0.0.xxx), and transfer speed was much higher.

The only drawback is when the two cables (Ethernet and Firewire) are
plugged together in the computer : I can't 'choose' which interface
the data should take, and files are transferred through the ethernet
network at the slower speed, instead of flowing directly to the other
computer through the firewire connection...

The only solution I have until now is to manually disable the RJ-45
interface (either through the network properties of Windows or by
removing the cable), but I soon need to switch it on again to access
the Internet.

How could I do to tell Windows which network I want to use to transfer
files ?

Any help welcome.

Thanks.