Once you change this on your computer you will no longer be able to use the computer to connect to your router through the same network adapter until you've undone all this.
An easier way is just click start, run, type "cmd" , hit OK. Now you're at a command prompt. type "ipconfig" and hit enter. This will tell you your PC's IP address. Now you just have to assign the Xbox an IP address on the same network, which means that the first three sections of the IP address must be identical. So:
if your PC's IP address is 192.168.0.5, assign the Xbox 192.168.0.6
if your PC's IP address is 192.168.1.10, assign the Xbox 192.168.1.11
if your PC's IP address is 10.0.0.100, assign the Xbox 10.0.0.200
The only numbers that should be different are the ones after the last decimal point, and they can be anything between 1 and 255. All the other numbers should match exactly between the Xbox IP address and the PC IP address.
The only way this won't work is if, when you type "cmd" at the command prompt on the PC, it tells you that your address is 0.0.0.0, in which case you will need to assign a static address to both the PC and the Xbox. In that case Malenko's instructions will probably get you there.
Here are some more complete instructions, with pictures.