(Old version, see XeXMenu below)
Updated 17/01/2010
Download XeXLoader ISO.rar and XeXLoader LIVE.rar ( LINK )
Burn XeXLoader.iso to a CD
Copy C0DE9999 to USB stick
Put USB stick in Xbox 360
Put XeXLoader disc in Xbox 360
Use D-Pad left and Right to navigate to USB stick
Press Y to bring up menu
Copy C0DE9999
Press X to bring up device menu
Navigate to
hdd:\Content\0000000000000000\
If the 0000000000000000 folder is not there, create it (16 zeros)
Paste C0DE9999 into 0000000000000000
Exit XeXLoader via the guide button.
XeXLoader should now be available from the Game Libary, so you don’t need the XeXLoader disc anymore 
do i need a jtag?
Yes you do.
my xbox 360 is not booting up. I think the xex menu has gone and the power is only signilling green on both powerbrick and xbox. No Xbox Rings But Sometimes it comes with 2 RROD. Please Help And Reply IF I Have To Reinstall XEX Menu Or Not.