Hi,
First of all, please understand that Microsoft will make every reasonable effort to assist you in troubleshooting home or peer-to-peer networking issues. However, if you use crossover cables when you configure your home or peer-to-peer network, Microsoft does not guarantee that issues will be resolved.
Using Crossover Cables in Home or Peer-to-Peer Networkshttp://support.microsoft.com/default.aspx?scid=KB;EN-US;278870
Before creating Internet Connection Sharing (ICS), please make sure that the Ethernet adapter does not have a default gateway. In Windows Vista only one gateway works at the same time. If you have set another Default Gateway in the connection for the Ethernet adapter, the Default Gateway of the computer will be changed when you plug the Ethernet crossover cable and the wireless Internet connection will not work.
In the properties of IPv4 protocol for the Ethernet adapter, do not set to obtain IP address automatically, and make sure that the Default Gateway is null.
Regarding Default Gateway, please refer the following articles.
Default Gateway Configuration for Multihomed Computershttp://support.microsoft.com/kb/157025
The IP routing table: TCP/IPhttp://technet.microsoft.com/en-us/library/cc779122.aspx
To make a point to point connection and share Internet via the Windows Vista computer, you need to share the network adapter of the Windows Vista computer.
How to share network adapter.=======================
1. Click Start. In the Start Search box type "ncpa.cpl" and press Enter. 2. Right-click on the wireless connection for Internet, click "Properties". 3. Click the tab "Share". Enable "Internet Connection Sharing".
After sharing the adapter, the IP address of the Local Area Connection will be change to 192.168.0.1. I suggest that you change the IP address to 192.168.0.2 or other addresses (because the address of the router on another floor may be 192.168.0.1).
Then, on the Windows XP computer, manually set the IP address. Make sure that the IP addresses of the Windows XP computer and Ethernet adapter on the Windows Vista computer are in the same network section. Then, on the Windows XP computer, set default gateway to the IP address of the Ethernet adapter of the Windows Vista computer.
If the above steps do not work, I suggest that you purchase a wireless network adapter for the Windows XP computer as a final solution.
Hope it helps
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