Have you tried updating via USB?
Latest update is available here
http://www.xbox.com/system-update-usbExtract the files to the root of a USB stick formatted to fat32 then restart the console
Failing that you can always update it by using a hard drive from a fully upto date Xbox 360 plugging that into your 360 and it will update itself automatically ive just updated a banned Xbox 360 that ive had for a while and it worked!! Since the update files are stored on the hard drive then used to flash the 360s internal flash memory aslong as the 360 hasnt had its cache cleared the update files will still be on the hard drive
![Very Happy :D](./images/smilies/icon_e_biggrin.gif)
This was done on the old style 360 not the new one that uses a 3.5 inch hard drive instead of the sideways brick so cant say if it will work on those as im not sure if they have changed the architecture or not