The xbox should be set to obtain it's IP automatically (DHCP)
if you are 100% sure you are getting the wireless key correct then it's possible that either DHCP isn't turned on on your router so it won't give out an IP address to the xbox, or that mac address filtering is turned on on the router so only known devices can connect.
think the best thing is to checkk a couple of setting on the router.
Open a web browser on a computer that is connected to the router and type the following in the address bar (or just click the link below)
http://192.168.1.1
you should get a prompt for a username and password, enter "virginmedia" for both
then when the routers admin pages have loaded click on the LAN IP settings link on the left and make sure the "use router as DHCP server has a tick in it.
then click on the wireless settings under advanced and select the setup access list button (this part is from memory) and in there set it so all devices are allowed to connect (doing this is not a security risk at all)
Then try and connect the xbox again (making sure it is set to obtain IP automatically)