Ok here we go, I am about to talk you through setting your router settings so that your xbox should work, but before you do this you need to turn on your xbox and set it to use a static IP (I dont know how right now but maybe Nodd can tell you, if not I will find out how and let you know). Once you find out how to set your xbox to a static IP I suggset you use "192.168.0.150" and will assume you are using this IP in the rest of this guide.
Step 1: First we need to login to your router, to do this open your internet browser (any browser will do but the most common are internet explorer or frirefox, I use firefox). Then in the browser address bar enter your routers IP address, which in this case is 192.168.0.1, and then press enter:

Step 2: You will be presented with a login window asing for username and password. For username enter "admin" (without quotes) and for the password enter "password" (again without the quotes).
Step 3: You should now be logged in to your router and have access to its settings as shown below:

Now choose the Services option.
Step 4: We now need to add some new services to your router so it will allow your xbox to play with other xboxs over xbox live. Click the "Add Custom Service" button:

Step 5: Now you will be presented with a screen that allows you to setup a new services. For the first service enter
Name: xbox1
Type: UDP
Start Port: 88
End Port: 88
Then click apply, this will add the rule to your router. Now follow the same process to add two more services using the following details:
Name: xbox2
Type: UDP
Start Port: 3074
End Port: 3074
Name: xbox3
Type: TCP
Start Port: 3074
End Port: 3074
Now all your rules are setup. But we havent finished yet im afraid.
Step 6: Now that we have setup your services we have to actully setup rules for your firewall that will use these services. On the left click firewall rules, this will open the firewall page as shown below (I already have some rules setup for diffrent things please dont let this confuse you):

We need to add our services to both the Outbound and Inbound tables. First lets add them to the Outbound table. To do this click the "Add" button.
Step 7: Now you will have the following screen:

Click on the Service dropdown box and choose the "xbox1" service we created eralier.
Then set the action dropdown box to "ALLOW always".
I suggest you leave the rest of your settings at the defaults for now.
Now press the apply button and repeat the process to add the other services we created before, xbox2 and xbox3.
Step 8: Now we need to add our Inbound rules:
Click on the Service dropdown box and choose the "xbox1" service we created eralier.
Then set the action dropdown box to "ALLOW always".
Set Send to LAN Server: to "192.168.0.150"
I suggest you leave the rest of your settings at the defaults for now.
Now press the apply button and repeat the process to add the other services we created before, xbox2 and xbox3.
Step 9: Now look to your left where all the options are, you may need to scroll down untill you find UPnP, click on it and it will bring up a new screen. make sure this screen has Turn UPnP On ticked.
Now you are done. Try xbox live and see if eveything is working ok. I hope it is

I think there are some spelling mistakes and I might have not put in enough detail but I need to sleep! I will edit this if you need more info/help.
ARGGG JUST SEEN THAT ITS MESSED UP MU IMAGE QUALITY! Sorry hope you can make it out. if not I will do more.