Vista - Multiple networks

Hello,

Some days ago, I tried to set up a network with a friend. I have 2 problems :

1) Sharing files
I have a Desktop computer with Vista & Internet access through RJ45 Straight (building network). My friend has a laptop running XP. We connected our computers with RJ45 crossover.
We reached to create a LAN between our PC but we couldn't share any file ! Here is the configuration we used (not usual, but worked!) :

PC Vista :
IP: 1.2.3.4
Subnet mask: 255.255.255.248
Gateway: 1.2.3.4

Laptop XP :
IP: 1.2.3.5
Subnet mask: 255.255.255.48
Gateway: 1.2.3.4

We had the same network group name.

When scanning the computers of its same network group, my friend's computer saw a machine with a strange name, which was not mine. That machine was sharing printers, whereas that option was not activated on my machine... but he couldn't see the file I was sharing.

2) Using a network bridge with MAC modifier
The building network I'm using has stickies on its switches ports (Cisco). Each port allows specific MAC address and shut the line if another MAC is detected.
When I set up a network bridge between the connexion with my friend's laptop and the connexion through which I connect to the Internet, as planned, my line has been shut.
Do you know a way to specify a new MAC address for the bridge?