to share 2 computers with one internet connection is an Easy Task on windows vista
You Can do this in two easy steps

step One
Make sure that the two computers have the same wokgroup name
for Example if the first computer have workgroup name HOME NETWORK Then the second computer should have the same name HOME NETWORK

To change the workgroup name
Right Click on My Computer then choose properties ,
from the system properties window choose Advanced system setting from the left panal

Select the Computer Name Tab then click on change bottom
Change the workgroup name as you want it to be then click ok

Step Two
open Network and Sharing Center
Start/Control Panel / Network and Sharing Center

on the left side choose Manage Network Connection

Select the internet connection you using then Right Click and Choose properties

Choose the Sharing Tab then select Allow other network users to connect thought this computer’s internet connection

then click ok

now the second computer share the internet connection through the first computer