Did you know? Xbox Live Account Management
Go to www.xbox.com and sign in with the Windows Live Passport you used when you setup Xbox Live (XBL) on you Xbox 360.
Now there are a couple ways to get to Account Management. Either Click on My Xbox next to your gamerpicture and then click on Manage Profile, or click on the little double arrow right next to My Xbox and select Manage Profile from the drop down menu.
From your computer, which has a wonderful sized keyboard and mouse, you can add MS Points, or Pre-paid XBL codes, and also update pretty much everything that you can about your Xbox Profile. So much easy on the computer then trying to enter a 5×5 (five by five, the 25 character alphanumeric code used for pretty much everything XBL related) via the controller.