Modem turns off My computer automatically

[YellowSnow]

Hi, The problem i'm having includes 2 computers, 2 ethernet cables, and 1 modem.

One of the computers runs windows vista and the one i'm having a problem with is the "emachine T3256" which runs windows xp.

When i connect the vista computer to a ethernet cable then to the modem i add an additionalethernet cable to the open slot on the modem and then connect it to the "emachine T3256"

Heres were the problem occurs, When i turn on the modem, about 30 secounds later the "emachine T3256" screen turns black but the computer is still powered but i'm not able to do anything but hold down the power button to turn it off manually.

note (the computers works fine when the modem is turned off)

I was just messing around though and have pinpointed the problem to be related with my NIVIDIA nForce MCP Networking Controller.

I've checked windows updates for updates and already have the most recent.

The Funny thing is that i have done this before with a older compaq computer instead of the
newer "emachine T3256"

Just so you know im not trying to split a internet connection between to 2 computers, but only to create a local area connection.

The modem i'm using is a Actiontec GT704WG which has worked before the new computer "emachine T3256" was added.

====ANY HELP AT ALL WOULD BE EXTREMELY APPRETIATED====

System Specs...
comp. thast works fine.
Windows Vista sp1
Gateway GT5654
AMD Athlon 64 X2
Processor 5000+
Operates at 2.6 GHz, 2x512 KB L2
Cache, 2000 MHz System bus
3 GB DDR2 Dual-Channel

Comp. thats not working.
Windows xp sp3
emachines T3256
AMD Athlon XP 3200+
Operates at 2.2 GHz
448 MB of RAM